-----Original Message----- From: Joakim Tjernlund [mailto:joakim.tjernlund@transmode.se] Sent: Friday, December 05, 2014 10:46 AM To: Diego Santa Cruz Cc: u-boot@lists.denx.de Subject: RE: [U-Boot] [PATCH 00/18] Support for eMMC partitioning and related fixes
[snip]
Write size should always be in a multiple of the superpage size to
maximize performance and life, and the superpage size will be a multiple of 4 KB for any reasonable eMMC device. Currently, as far as I can tell, the mmc code does the writes in multiple blocks, by chunks of up to CONFIG_SYS_MMC_MAX_BLK_COUNT (65535 by default) 512-byte blocks, so data is written in sufficiently big chunks as long as the mmc write command is issued properly aligned and with a proper length. The eMMC device does not have to have committed the data until the end of the multiple block write command, so it should buffer up the data to sufficiently large buffers before writing them to NAND, lifting most of the performance penalty of the 512-byte sectors and, I believe, lifting all the life penalty. For a reliable write the situation may be a bit different.
Yes, reliable write is the key(which we need, doesn't everybody? :). This is even default for some eMMC devices.
This being said, I realized that switching to the native sector size is
a one-time operation that needs to be done before hardware partitioning, so yes it would be good to add it to U-Boot. But, as I said, I do not have any 4.5 / 4.51 devices at hand to develop this feature with.
You could just "copy" the code from mmc-utils and do a RFC impl. with a big fat warning attached to it.
Then I leave it as an exercise for the reader :-) Note that after setting a 4 KB device to its native sector size you cannot issue any read nor write whose offset or size is not 4KB aligned, I do not know if all U-Boot code is currently capable of this, otherwise the mmc read / write code needs to emulate unaligned accesses.
While writing this I also realized that the current choice of
CONFIG_SYS_MMC_MAX_BLK_COUNT being 65535 is not good, as writing more than 65535 512-byte blocks will result in issuing non-aligned writes on the second and subsequent chunks when writing 32 MiB or more in a single command. A simple fix for this would be to set the default value for CONFIG_SYS_MMC_MAX_BLK_COUNT to 65280 (i.e. aligned to 256 KiB) so that it is aligned to any reasonable superpage size. A more elaborate fix would be for b_max to be rounded down to a multiple of the superpage size read from EXT_CSD in mmc_startup().
yes, you could consider 4 MiB alignment as well, http://lwn.net/Articles/428584/
It seems to me that 4 MiB alignment for breaking up a large write into chunks is overkill. It is the user of the eMMC that should take care to align things to 4 MiB or similar. I guess it would be even better to align the things to the write protect group or erase size.
I'll cook something up for this issue as soon as I have some spare time.
Best,