linux-kernel - Re: [PATCH] mmc: core: Remove timeout when enabling cache

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Date:   Tue, 20 Nov 2018 15:24:27 +0100
From:   Ulf Hansson <ulf.hansson@...aro.org>
To:     Sjoerd Simons <sjoerd.simons@...labora.co.uk>
Cc:     Wolfram Sang <wsa@...-dreams.de>, Faiz Abbas <faiz_abbas@...com>,
        "linux-mmc@...r.kernel.org" <linux-mmc@...r.kernel.org>,
        kernel@...labora.com,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Hongjie Fang <hongjiefang@...micro.com>,
        Bastian Stender <bst@...gutronix.de>,
        Kyle Roeschley <kyle.roeschley@...com>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Shawn Lin <shawn.lin@...k-chips.com>,
        Harish Jenny K N <harish_kandiga@...tor.com>,
        Simon Horman <horms+renesas@...ge.net.au>,
        Hal Emmerich <hal@...emmerich.com>
Subject: Re: [PATCH] mmc: core: Remove timeout when enabling cache

On 20 November 2018 at 15:00, Sjoerd Simons
<sjoerd.simons@...labora.co.uk> wrote:
> On Tue, 2018-11-20 at 14:08 +0100, Ulf Hansson wrote:
>> + Hal Emmerich
>>
>> On 20 November 2018 at 12:38, Sjoerd Simons
>> <sjoerd.simons@...labora.co.uk> wrote:
>> > On Tue, 2018-11-20 at 11:23 +0100, Wolfram Sang wrote:
>> > > > > >
>> > So if you know the pattern, or just happen to hit it often in e.g.
>> > automated testing, it does show up during development. Otherwise it
>> > can
>> > appear to "happen once in a while randomly".
>>
>> I don't quite follow. As far as I understand, the extended timeout is
>> needed when turning the cache on.
>>
>> The above seems more related to flushing the cache, no? Flushing have
>> no timeout (also reported to be an issue [1]), which happens either
>> at
>> _mmc_hw_reset() or at _mmc_suspend().
>>
>> What is the relation here?
>
> Yes it's the kinda of behaviour you would expect on a flush indeed! I
> don't know what the card actaully does when turning the cache on,
> whether it's actually flush of something persistent when turning the
> cache on after a hard poweroff or doing some other validation.
>
> All i can share is what our testing seems to indicate, which is that
> there is a wide spread in the time the card needs *and* there seems to
> be strong correlation to the I/O activity before the hard power off and
> the time taken by "cache on".

So the hard power off, means that you are cutting the power to the
platform and not doing a graceful power off? Just so I understand
correctly.

>
>> > Unfortunately for me, it was really a case of getting reports of
>> > some
>> > boards started failing at some point which took a while to track
>> > back.
>> > Especially since it's a battery powered device (thus hard poweroffs
>> > are
>> > rather rare) and we allow the board manufactorer to select from
>> > various
>> > different eMMCs depending on price/available at build time...
>> >
>> > > Yet, if we add a quirk for that, then we should probably mention
>> > > it
>> > > in
>> > > an error message when we hit -ETIMEDOUT for cache on ("does your
>> > > card
>> > > need this quirk?")? It can be pretty time consuming to track this
>> > > down
>> > > otherwise, I'd think.
>> >
>> > Yes please. It would be nice if someone happens to have the right
>> > contacts with Micron to see if it's a known issue for their cards
>> > in
>> > general or just this one.
>> >
>> > Also would be good to have a timeout higher then 1 seconds (or for
>> > these cards not have one?); On our testing thusfar we've seen
>> > timeouts
>> > up to 850ms, but it's impossible to ensure that that's the true
>> > upper
>> > bound.
>>
>> Using no limit of the timeout, would mean we may hang for ~10 minutes
>> (MMC_OPS_TIMEOUT_MS) instead, no thanks.
>
> Probably a silly question, but would this actually cause e.g. boot to
> hang while waiting for the card (assuming rootfs is somewhere else)?

Nope.

[...]

Kind regards
Uffe