Re: [PATCH v3 0/9] Add support for segments smaller than one page

[Bart Van Assche <bvanassche@acm.org>]

On 1/18/23 17:18, Jens Axboe wrote:
> On 1/18/23 3:54?PM, Bart Van Assche wrote:
>> Hi Jens,
>>
>> Several embedded storage controllers need support for DMA segments that are
>> smaller than the size of one virtual memory page. Hence this patch series.
>> Please consider this patch series for the next merge window.
> 
> Before any real reviews are done, I have to ask "why?". This is pretty
> hairy code in the middle of the fast path for some obscure controller.
> Why would anyone ship that with > 4k page sizes rather than ship it with
> a controller that is sane?

Hi Jens,

The new config variable CONFIG_BLK_SUB_PAGE_SEGMENTS has been introduced 
to make sure that the hot path is *not* affected if that config variable 
is disabled.

Regarding the question "why": the Google Android team would like to 
improve performance by switching from 4 KiB pages to 16 KiB pages. One 
of the widely used UFS controllers (Exynos) has a maximum segment size 
of 4 KiB. Hence this patch series.

This patch series is not only useful for phones but also for Tesla cars 
since Tesla cars use an Exynos UFS controller.

A contributor to the MMC driver told me that this patch series would 
allow to simplify the MMC driver significantly (I have not yet double 
checked this).

Bart.