Re: question about block layer and dma alignment issue.

[Oleksij Rempel <linux@rempel-privat.de>]

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Am 04.10.2014 um 19:00 schrieb Jens Axboe:
> On 2014-10-04 00:59, Oleksij Rempel wrote:
>> Hello Jens,
>>
>> i hope it is OK to ask you directly, i didn't found better way to do
>> this.
>> I'm writing MMC driver based on reverse engineering for au6601 mmc
>> controller. This driver uses MMC API, and last one trying to use BLK API
>> as directly as possible. So far so gut, but i have a problem by using SG
>> requests with DMA without conversation. This controller accepts only
>> addresses with this mask 0xffff.f000 - also page aligned.
>> Till now i tried different QUEUE_FLAG_ flags and blk_queue settings, but
>> i never had absolute aligned quarantine.
>> Please tell me if i do some thing wrong or go in the wrong direction.
> 
> Unless you can accept a 4kb hardware block size (and I'm assuming you
> cannot, since you need to support 512b file systems?), then you'd have
> to bounce the requests that don't align properly. You can fix some of
> them by setting blk_queue_dma_alignment(q, 4095).

I needed to include MMC list with Ulf Hansson at the beginning, so here
we are.

Hi Ulf,

here is the list of drivers which do care about dma alignment . Some of
them use bounce_buffer, some will use PIO instead of DMA, for not
aligned requests:

dw_mmc.c: dw_mci_pre_dma_transfer
wbsd.c
usdhi6rol0.c
tmio_mmc_dma.c
sunxi-mmc.c
sdhci.c
pxamci.c
mvsdio.c
au6601.c(not upstream jet)

The problem of all this driver is that each of them use own solution. I
think this code can be moved to some common place. For example, it can
be done in drivers/mmc/card/queue.c and enabled by
mmc_host_dma_alignment variable. Or provide a mmc_sg_align() function,
which should be called explicatively by driver.

Which option is preferable?

-- 
Regards,
Oleksij


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