linux-kernel - Re: [PATCH v2] mmc: meson-gx: fix mmc dma operation

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Message-ID: <1jeeydf27h.fsf@starbuckisacylon.baylibre.com>
Date:   Tue, 12 Nov 2019 18:12:50 +0100
From:   Jerome Brunet <jbrunet@...libre.com>
To:     Nan Li <Nan.Li@...ogic.com>, Jianxin Pan <Jianxin.Pan@...ogic.com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Kevin Hilman <khilman@...libre.com>
Cc:     Neil Armstrong <narmstrong@...libre.com>,
        "linux-amlogic\@lists.infradead.org" 
        <linux-amlogic@...ts.infradead.org>,
        "linux-mmc\@vger.kernel.org" <linux-mmc@...r.kernel.org>,
        "linux-kernel\@vger.kernel.org" <linux-kernel@...r.kernel.org>,
        Victor Wan <victor.wan@...ogic.com>
Subject: Re: [PATCH v2] mmc: meson-gx: fix mmc dma operation


On Wed 06 Nov 2019 at 04:28, Nan Li <Nan.Li@...ogic.com> wrote:

>
> I see what you mean, pre/post_req already provides callbacks in meson_mmc_ops for the framework to decide whether to invoke the chain mode or not.
>
> However, I searched the frame of MMC and found the use of mmc_pre_req() for this callback in the block layer mmc_blk_mq_issue_rw_rq().
>
> Block layer mmc_blk_mq_issue_rw_rq() may be useful for emmc and SD card devices.
>
> But it may not be useful for reading and writing operations of sdio wifi, and the sdio device communication may not use the chain mode.
>
>
> Our chain-mode is a way to transfer data using dma.
>
> This approach is very efficient for reading and writing large amounts of data.
>
> If you don't do it that way, you'll do it the other way, the bounce buf way, which is limited by the size of the buf, so when you do big data reads and writes, it affects the transfer rate.
>
> Therefore, our chain mode means that emmc, SD card or sdio device will use dma to transfer data when reading and writing operations, so our previous driver and the patch behind me all ensure this.
>

I see.
The problem is that you are providing the same function to pre/post_req
callbacks and the request()

IOW, things mapped in the pre_req() callback might be unmapped by
request_done() instead post_req() which, I think, is not great.

It's been like that so far, your patch is not making much worse, so I
guess you can go ahead with it but we need to look a this before it
blows again

In the future, we should probably use the cookie to distinguish the 2
cases ... or drop pre/post_req in the ops.

Regards
Jerome