linux-kernel - MMC/SDIO sub-system: block mode versus byte mode

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Date:	Thu, 22 Nov 2007 10:41:43 +0000
From:	Dean Jenkins <djenkins@...sta.co.uk>
To:	Pierre Ossman <drzeus-mmc@...eus.cx>
Cc:	Linux kernel mailing list <linux-kernel@...r.kernel.org>
Subject: MMC/SDIO sub-system: block mode versus byte mode

Hi Pierre,

I've been checking my SDIO CMD53 data transfers and I notice the
following:

block size = 256 bytes for my SDIO card.

transfers of 256 bytes uses byte mode ( I expected block mode )
transfers of 512 bytes uses block mode ( because blocks = 2 )

>>From the SD spec a single block transfer seems to be valid.

I notice the function mmc_io_rw_extended() in sdio_ops.c seems to select
byte mode when the transfer size is 512 or less when the blocks
parameter variable is 1. In fact blocks is never 0.

Therefore is this a feature or a bug that prevents 256 bytes being sent
in block mode when it is a single complete 1 block of data ?

If it is a bug then I suggest the sdio_io_rw_ext_helper() in sdio_io.c
be able to select byte mode by setting blocks to 0 and then blocks can
be set to 1 to select block mode for a single block.

Have I missed something ?

Technically, there is a bug in sdio_io_rw_ext_helper() in sdio_io.c

211		while (remainder > func->cur_blksize) {

should be, = missing

211		while (remainder >= func->cur_blksize) {

however the resulting call to mmc_io_rw_extended() is the same as blocks
= 1 for both the "block mode" while loop (with the "fix") and the "byte
mode" while loop (without the "fix"). The "byte mode" while loop has a
hard coded value of 1 for the blocks parameter. So the bug is masked. As
I said above, I think the "byte mode" should use a value of 0 for the
blocks parameter.

What do you think ?

Regards,
Dean. 

-- 
Dean Jenkins
Embedded Software Engineer
MontaVista Software (UK)
Professional Services Division

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