linux-kernel - Re: [PATCH] spi: Add a sysfs interface to instantiate devices

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Date:   Sat, 23 Dec 2017 09:58:51 +0100
From:   Geert Uytterhoeven <geert@...ux-m68k.org>
To:     Kyle Roeschley <kyle.roeschley@...com>
Cc:     Mark Brown <broonie@...nel.org>, Trent Piepho <tpiepho@...inj.com>,
        "linux-spi@...r.kernel.org" <linux-spi@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] spi: Add a sysfs interface to instantiate devices

Hi Kyle,

On Fri, Dec 22, 2017 at 6:11 PM, Kyle Roeschley <kyle.roeschley@...com> wrote:
> On Fri, Dec 22, 2017 at 03:56:03PM +0000, Mark Brown wrote:
>> On Thu, Dec 21, 2017 at 09:05:43PM +0000, Trent Piepho wrote:
>> > On Thu, 2017-12-21 at 14:03 -0600, Kyle Roeschley wrote:
>> > > Add a sysfs interface to instantiate and delete SPI devices using the
>> > > spidev driver. This can be used when developing a driver on a
>> > > self-soldered board which doesn't yet have proper SPI device declaration
>> > > at the platform level, and presumably for various debugging situations.
>>
>> > > Inspired by 99cd8e25875a ("i2c: Add a sysfs interface to instantiate
>> > > devices").
>>
>> > The i2c interface allows one to specify the type of device to create.
>> > Why must this interface be linked to spidev and only capable of
>> > creating spidev devices?
>>
>> Right, that doesn't seem good.  I also can't see anything in the actual
>> code which suggests that this is tied to spidev except the log messages.
>
> Quoting Geert's email [1] on the subject:
>
>> To me, the above sounds a bit contradictive: either you have
>>   1. a simple (trivial) description, which can be handled by spidev and
>>      userspace, and thus by just writing "<unit-addr> spidev" to a new_device

Note the "spidev" in the string written...

>>      sysfs node, or
>>   2. a complex description, for which you need a specialized in-kernel driver,
>>      so you're gonna need a real DT node (and overlays?) to describe it.
>>
>> I don't think writing a complex description to a new_device sysfs node makes
>> sense.
>
> And regarding not being linked to spidev, see modalias in new_device_store:
>
>> > > + struct spi_board_info bi = {
>> > > +         .modalias = "spidev",

I would make it a little bit more generic and extract the modalias from the
string written.

>> > > +         .max_speed_hz = ctlr->max_speed_hz,
>> > > + };
>
> [1] https://marc.info/?l=linux-spi&m=151199390921251&w=2

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds