linux-kernel - Re: [PATCH 0/2] mmc: allow mmc_alloc_host() and tmio_mmc_host

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Date:   Fri, 11 Nov 2016 12:19:05 +0900
From:   Masahiro Yamada <yamada.masahiro@...ionext.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     linux-mmc <linux-mmc@...r.kernel.org>,
        Jarkko Lavinen <jarkko.lavinen@...ia.com>,
        Rui Miguel Silva <rmfrfs@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Nicolas Pitre <nico@...xnic.net>, devel@...verdev.osuosl.org,
        Jaehoon Chung <jh80.chung@...sung.com>,
        Pierre Ossman <pierre@...man.eu>,
        Johan Hovold <johan@...nel.org>, Alex Dubov <oakad@...oo.com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Tony Olech <tony.olech@...ndigitalsystems.com>,
        Adrian Hunter <adrian.hunter@...el.com>,
        Michał Mirosław <mirq-linux@...e.qmqm.pl>,
        Sascha Sommer <saschasommer@...enet.de>,
        adi-buildroot-devel@...ts.sourceforge.net,
        Ben Dooks <ben-linux@...ff.org>, Alex Elder <elder@...nel.org>,
        "moderated list:ARM/Mediatek SoC support" 
        <linux-mediatek@...ts.infradead.org>,
        Wolfram Sang <wsa+renesas@...g-engineering.com>,
        Tony Prisk <linux@...sktech.co.nz>, linux-omap@...r.kernel.org,
        Manuel Lauss <manuel.lauss@...il.com>,
        Russell King <linux@...linux.org.uk>,
        linux-arm-kernel <linux-arm-kernel@...ts.infradead.org>,
        Ulf Hansson <ulf.hansson@...aro.org>,
        Ludovic Desroches <ludovic.desroches@...el.com>,
        linux-usb@...r.kernel.org, Bruce Chang <brucechang@....com.tw>,
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        Harald Welte <HaraldWelte@...tech.com>,
        Chen-Yu Tsai <wens@...e.org>,
        Maxime Ripard <maxime.ripard@...e-electrons.com>
Subject: Re: [PATCH 0/2] mmc: allow mmc_alloc_host() and tmio_mmc_host_alloc()

2016-11-10 22:35 GMT+09:00 Greg Kroah-Hartman <gregkh@...uxfoundation.org>:
> On Thu, Nov 10, 2016 at 10:24:21PM +0900, Masahiro Yamada wrote:
>>
>> sdhci_alloc_host() returns an error pointer when it fails.
>> but mmc_alloc_host() cannot.
>>
>> This series allow to propagate a proper error code
>> when host-allocation fails.
>
> Why?  What can we really do about the error except give up?  Why does
> having a explicit error value make any difference to the caller, they
> can't do anything different, right?


The error code is shown in the console, like

  probe of 5a000000.sdhc failed with error -12


The proper error code will give a clue
why the driver failed to probe.


> I suggest just leaving it as-is, it's simple, and you don't have to mess
> with PTR_ERR() anywhere.


Why?

Most of driver just give up probing for any error,
but we still do ERR_PTR()/PTR_ERR() here and there.
I think this patch is the same pattern.

If a function returns NULL on failure, we need to think about
"what is the most common failure case".

Currently, MMC drivers assume -ENOMEM is the best
fit for mmc_alloc_host(), but the assumption is fragile.

Already, mmc_alloc_host() calls a function
that returns not only -ENOMEM, but also -ENOSPC.

In the future, some other failure cases might be
added to mmc_alloc_host().

Once we decide the API returns an error pointer,
drivers just propagate the return value from the API.
This is much more stable implementation.



-- 
Best Regards
Masahiro Yamada