linux-kernel - Re: [PATCH v6 0/4] Add MMC software queue support

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Date:   Fri, 22 Nov 2019 14:49:42 +0100
From:   Arnd Bergmann <arnd@...db.de>
To:     Linus Walleij <linus.walleij@...aro.org>
Cc:     "(Exiting) Baolin Wang" <baolin.wang@...aro.org>,
        Baolin Wang <baolin.wang7@...il.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Asutosh Das <asutoshd@...eaurora.org>,
        Orson Zhai <orsonzhai@...il.com>,
        Lyra Zhang <zhang.lyra@...il.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        linux-mmc <linux-mmc@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Hannes Reinecke <hare@...e.com>,
        linux-block <linux-block@...r.kernel.org>,
        Paolo Valente <paolo.valente@...aro.org>
Subject: Re: [PATCH v6 0/4] Add MMC software queue support

On Fri, Nov 22, 2019 at 2:20 PM Linus Walleij <linus.walleij@...aro.org> wrote:
>
> On Fri, Nov 22, 2019 at 10:50 AM Arnd Bergmann <arnd@...db.de> wrote:
>
> > I suppose to make the submission non-blocking, all operations that
> > currently block in the submission path may have to be changed first.
> >
> > For the case of a partition switch (same for retune), I suppose
> > something like this can be done:
> >
> > - in queue_rq() check whether a partition switch is needed. If not,
> >   submit the current rq
> > - if a partition switch is needed, submit the partition switch cmd
> >   instead, and return busy status
> > - when the completion arrives for the partition switch, call back into
> >   blk_mq to have it call queue_rq again.
> >
> > Or possibly even (this might not be possible without signifcant
> > restructuring):
> >
> > - when preparing a request that would require a partition switch,
> >   insert another meta-request to switch the partition ahead of it.
> >
> > I do realize that this is a significant departure from how it was done
> > in the past, but it seems cleaner that way to me.
>
> This partition business really need a proper overhaul.
>
> I outlined the work elsewhere but the problem is that the
> eMMC "partitions" such as boot partitions and the usecase-defined
> "general" partition (notice SD cards do not have this problem)
> are badly integrated with the Linux partition manager.

I think that's a totally orthogonal problem though: we may
well be able to come up with a different way of representing
the extra partitions to user space or as separate block devices,
but in the end, this looks exactly the same to mm
 ->queue_rq() callback. If we have send a cmd to one partition
and want to send the next cmd to another partition, we first
have to send the partition switch cmd.

      Arnd