Fixed MTP to work with TWRP

Documentation/mmc/00-INDEX

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+00-INDEX
+        - this file
+mmc-dev-attrs.txt
+        - info on SD and MMC device attributes
+mmc-dev-parts.txt
+        - info on SD and MMC device partitions
+mmc-async-req.txt
+        - info on mmc asynchronous requests

Documentation/mmc/mmc-async-req.txt

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+Rationale
+=========
+
+How significant is the cache maintenance overhead?
+It depends. Fast eMMC and multiple cache levels with speculative cache
+pre-fetch makes the cache overhead relatively significant. If the DMA
+preparations for the next request are done in parallel with the current
+transfer, the DMA preparation overhead would not affect the MMC performance.
+The intention of non-blocking (asynchronous) MMC requests is to minimize the
+time between when an MMC request ends and another MMC request begins.
+Using mmc_wait_for_req(), the MMC controller is idle while dma_map_sg and
+dma_unmap_sg are processing. Using non-blocking MMC requests makes it
+possible to prepare the caches for next job in parallel with an active
+MMC request.
+
+MMC block driver
+================
+
+The mmc_blk_issue_rw_rq() in the MMC block driver is made non-blocking.
+The increase in throughput is proportional to the time it takes to
+prepare (major part of preparations are dma_map_sg() and dma_unmap_sg())
+a request and how fast the memory is. The faster the MMC/SD is the
+more significant the prepare request time becomes. Roughly the expected
+performance gain is 5% for large writes and 10% on large reads on a L2 cache
+platform. In power save mode, when clocks run on a lower frequency, the DMA
+preparation may cost even more. As long as these slower preparations are run
+in parallel with the transfer performance won't be affected.
+
+Details on measurements from IOZone and mmc_test
+================================================
+
+https://wiki.linaro.org/WorkingGroups/Kernel/Specs/StoragePerfMMC-async-req
+
+MMC core API extension
+======================
+
+There is one new public function mmc_start_req().
+It starts a new MMC command request for a host. The function isn't
+truly non-blocking. If there is an ongoing async request it waits
+for completion of that request and starts the new one and returns. It
+doesn't wait for the new request to complete. If there is no ongoing
+request it starts the new request and returns immediately.
+
+MMC host extensions
+===================
+
+There are two optional members in the mmc_host_ops -- pre_req() and
+post_req() -- that the host driver may implement in order to move work
+to before and after the actual mmc_host_ops.request() function is called.
+In the DMA case pre_req() may do dma_map_sg() and prepare the DMA
+descriptor, and post_req() runs the dma_unmap_sg().
+
+Optimize for the first request
+==============================
+
+The first request in a series of requests can't be prepared in parallel
+with the previous transfer, since there is no previous request.
+The argument is_first_req in pre_req() indicates that there is no previous
+request. The host driver may optimize for this scenario to minimize
+the performance loss. A way to optimize for this is to split the current
+request in two chunks, prepare the first chunk and start the request,
+and finally prepare the second chunk and start the transfer.
+
+Pseudocode to handle is_first_req scenario with minimal prepare overhead:
+
+if (is_first_req && req->size > threshold)
+   /* start MMC transfer for the complete transfer size */
+   mmc_start_command(MMC_CMD_TRANSFER_FULL_SIZE);
+
+   /*
+    * Begin to prepare DMA while cmd is being processed by MMC.
+    * The first chunk of the request should take the same time
+    * to prepare as the "MMC process command time".
+    * If prepare time exceeds MMC cmd time
+    * the transfer is delayed, guesstimate max 4k as first chunk size.
+    */
+    prepare_1st_chunk_for_dma(req);
+    /* flush pending desc to the DMAC (dmaengine.h) */
+    dma_issue_pending(req->dma_desc);
+
+    prepare_2nd_chunk_for_dma(req);
+    /*
+     * The second issue_pending should be called before MMC runs out
+     * of the first chunk. If the MMC runs out of the first data chunk
+     * before this call, the transfer is delayed.
+     */
+    dma_issue_pending(req->dma_desc);

Documentation/mmc/mmc-dev-attrs.txt

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+SD and MMC Block Device Attributes
+==================================
+
+These attributes are defined for the block devices associated with the
+SD or MMC device.
+
+The following attributes are read/write.
+
+	force_ro		Enforce read-only access even if write protect switch is off.
+
+SD and MMC Device Attributes
+============================
+
+All attributes are read-only.
+
+	cid			Card Identifaction Register
+	csd			Card Specific Data Register
+	scr			SD Card Configuration Register (SD only)
+	date			Manufacturing Date (from CID Register)
+	fwrev			Firmware/Product Revision (from CID Register) (SD and MMCv1 only)
+	hwrev			Hardware/Product Revision (from CID Register) (SD and MMCv1 only)
+	manfid			Manufacturer ID (from CID Register)
+	name			Product Name (from CID Register)
+	oemid			OEM/Application ID (from CID Register)
+	prv			Product Revision (from CID Register) (SD and MMCv4 only)
+	serial			Product Serial Number (from CID Register)
+	erase_size		Erase group size
+	preferred_erase_size	Preferred erase size
+	raw_rpmb_size_mult	RPMB partition size
+	rel_sectors		Reliable write sector count
+
+Note on Erase Size and Preferred Erase Size:
+
+	"erase_size" is the  minimum size, in bytes, of an erase
+	operation.  For MMC, "erase_size" is the erase group size
+	reported by the card.  Note that "erase_size" does not apply
+	to trim or secure trim operations where the minimum size is
+	always one 512 byte sector.  For SD, "erase_size" is 512
+	if the card is block-addressed, 0 otherwise.
+
+	SD/MMC cards can erase an arbitrarily large area up to and
+	including the whole card.  When erasing a large area it may
+	be desirable to do it in smaller chunks for three reasons:
+		1. A single erase command will make all other I/O on
+		the card wait.  This is not a problem if the whole card
+		is being erased, but erasing one partition will make
+		I/O for another partition on the same card wait for the
+		duration of the erase - which could be a several
+		minutes.
+		2. To be able to inform the user of erase progress.
+		3. The erase timeout becomes too large to be very
+		useful.  Because the erase timeout contains a margin
+		which is multiplied by the size of the erase area,
+		the value can end up being several minutes for large
+		areas.
+
+	"erase_size" is not the most efficient unit to erase
+	(especially for SD where it is just one sector),
+	hence "preferred_erase_size" provides a good chunk
+	size for erasing large areas.
+
+	For MMC, "preferred_erase_size" is the high-capacity
+	erase size if a card specifies one, otherwise it is
+	based on the capacity of the card.
+
+	For SD, "preferred_erase_size" is the allocation unit
+	size specified by the card.
+
+	"preferred_erase_size" is in bytes.
+
+Note on raw_rpmb_size_mult:
+	"raw_rpmb_size_mult" is a mutliple of 128kB block.
+	RPMB size in byte is calculated by using the following equation:
+	RPMB partition size = 128kB x raw_rpmb_size_mult
+
+SD/MMC/SDIO Clock Gating Attribute
+==================================
+
+Read and write access is provided to following attribute.
+This attribute appears only if CONFIG_MMC_CLKGATE is enabled.
+
+	clkgate_delay	Tune the clock gating delay with desired value in milliseconds.
+
+echo <desired delay> > /sys/class/mmc_host/mmcX/clkgate_delay

Documentation/mmc/mmc-dev-parts.txt

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+SD and MMC Device Partitions
+============================
+
+Device partitions are additional logical block devices present on the
+SD/MMC device.
+
+As of this writing, MMC boot partitions as supported and exposed as
+/dev/mmcblkXboot0 and /dev/mmcblkXboot1, where X is the index of the
+parent /dev/mmcblkX.
+
+MMC Boot Partitions
+===================
+
+Read and write access is provided to the two MMC boot partitions. Due to
+the sensitive nature of the boot partition contents, which often store
+a bootloader or bootloader configuration tables crucial to booting the
+platform, write access is disabled by default to reduce the chance of
+accidental bricking.
+
+To enable write access to /dev/mmcblkXbootY, disable the forced read-only
+access with:
+
+echo 0 > /sys/block/mmcblkXbootY/force_ro
+
+To re-enable read-only access:
+
+echo 1 > /sys/block/mmcblkXbootY/force_ro
+
+The boot partitions can also be locked read only until the next power on,
+with:
+
+echo 1 > /sys/block/mmcblkXbootY/ro_lock_until_next_power_on
+
+This is a feature of the card and not of the kernel. If the card does
+not support boot partition locking, the file will not exist. If the
+feature has been disabled on the card, the file will be read-only.
+
+The boot partitions can also be locked permanently, but this feature is
+not accessible through sysfs in order to avoid accidental or malicious
+bricking.