linux-kernel - Re: [PATCH V8 01/14] mmc: core: Introduce host claiming by context

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Date:   Fri, 22 Sep 2017 13:23:48 +0200
From:   Linus Walleij <linus.walleij@...aro.org>
To:     Adrian Hunter <adrian.hunter@...el.com>
Cc:     Ulf Hansson <ulf.hansson@...aro.org>,
        linux-mmc <linux-mmc@...r.kernel.org>,
        linux-block <linux-block@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Bough Chen <haibo.chen@....com>,
        Alex Lemberg <alex.lemberg@...disk.com>,
        Mateusz Nowak <mateusz.nowak@...el.com>,
        Yuliy Izrailov <Yuliy.Izrailov@...disk.com>,
        Jaehoon Chung <jh80.chung@...sung.com>,
        Dong Aisheng <dongas86@...il.com>,
        Das Asutosh <asutoshd@...eaurora.org>,
        Zhangfei Gao <zhangfei.gao@...il.com>,
        Sahitya Tummala <stummala@...eaurora.org>,
        Harjani Ritesh <riteshh@...eaurora.org>,
        Venu Byravarasu <vbyravarasu@...dia.com>,
        Shawn Lin <shawn.lin@...k-chips.com>,
        Christoph Hellwig <hch@....de>
Subject: Re: [PATCH V8 01/14] mmc: core: Introduce host claiming by context

On Wed, Sep 13, 2017 at 1:40 PM, Adrian Hunter <adrian.hunter@...el.com> wrote:

> Currently the host can be claimed by a task.  Change this so that the host
> can be claimed by a context that may or may not be a task.  This provides
> for the host to be claimed by a block driver queue to support blk-mq, while
> maintaining compatibility with the existing use of mmc_claim_host().
>
> Signed-off-by: Adrian Hunter <adrian.hunter@...el.com>

I think this is a reasonable intermediate step as well.
I am working to kill off the "big MMC host lock" by the following
plan:

DONE:
- Convert all ioctl() ops to custom block layer requests (done)
- Convert all debugfs ops to custom block layer requests (done)
- Convert RPMB access to a character device to avoid one
  partition switch (just merged)

REMAINS:
- Convert boot and "general" partitions to be part of the main
  block device. Because they are. (This is really tricky and
  may require changes in the core kernel partition handling.)
- Delete the dual mode card support. (No-one seems to be
  using it.)
- Split the world in block access and SDIO with something
  like a simple mutex: either the host is a block device OR
  it is SDIO, and that state has the same lifetime as a card,
  since they are now either block devices or SDIO not both
  at the same time.
- Implement a lighter lock for SDIO host claiming.
- Now the lock should be gone...

As you can see it is not entirely trivial :/

Linus