Re: [PATCH 2/2] spi: imx: fix use of native chip-selects with devicetree

[Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>]

Hi Uwe,

On Tue, Mar 21, 2017 at 8:23 PM, Uwe Kleine-König
<u.kleine-koenig@xxxxxxxxxxxxxx> wrote:
> On Tue, Mar 21, 2017 at 11:22:27PM +1000, Greg Ungerer wrote:
>> On 21/03/17 22:11, Uwe Kleine-König wrote:
>> > On Tue, Mar 21, 2017 at 09:53:52PM +1000, Greg Ungerer wrote:
>> > > On 21/03/17 18:05, Uwe Kleine-König wrote:
>> > > > On Tue, Mar 21, 2017 at 12:05:20PM +1000, Greg Ungerer wrote:
>> > > > > On 20/03/17 23:22, Vladimir Zapolskiy wrote:
>> > > > > > For that type of bindings locally I have a hackish spi-imx driver change,
>> > > > > > which supports this option, but I'm unsure if it is universal enough.
>> > > > >
>> > > > > Do you mean supporting no cs-gpios tag?
>> > > > > That would be nice, but it would seem not many users of this are
>> > > > > using native chip selects.
>> > > >
>> > > > The reason for this is that the native chip selects are less flexible
>> > > > than gpios because you cannot control when they deassert. IIRC they do
>> > > > it too much for some chips. So the only reason to stick to them is that
>> > > > on some SoCs not all pins have a GPIO function. Not sure if transfer
>> > > > speed is another reason, but I would expect that the gain isn't that
>> > > > big.
>> > >
>> > > For the particular SPI device I am using, a Silicon Labs 32260,
>> > > it actually wants the assertion and de-assertion of the chip-select
>> > > between each byte. So it is the only way it can work for me.
>> >
>> > That should be doable with gpio-cs, too. You just need the right flags
>> > in your spi transfer IIRC.
>>
>> Do you know which flag(s)  do that?
>
> Looking at the source it's not about flags, but you have to split your
> transfer into several messages.

... and set spi_transfer.cs_change.

> AFAICT that's how the spi stuff is
> supposed to work. That is, at the start of a message CS is asserted and
> (only) at it's end CS is deasserted. So the imx core with native chip
> select actually misbehaves by toggling CS between each word.

Indeed.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@xxxxxxxxxxxxxx

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
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