Re: [PATCH 7/7] drm/panel: sitronix-st7789v: Check display ID

[Miquel Raynal <miquel.raynal@bootlin.com>]

Hi Sebastian,

sre@kernel.org wrote on Thu, 15 Jun 2023 01:27:24 +0200:

> Hi,
> 
> On Sat, Jun 10, 2023 at 10:45:25PM +0200, Sam Ravnborg wrote:
> > Hi Miquel,
> > 
> > On Fri, Jun 09, 2023 at 04:59:51PM +0200, Miquel Raynal wrote:  
> > > A very basic debugging rule when a device is connected for the first
> > > time is to access a read-only register which contains known data in
> > > order to ensure the communication protocol is properly working. This
> > > driver lacked any read helper which is often a critical peace for
> > > fastening bring-ups.
> > > 
> > > Add a read helper and use it to verify the communication with the panel
> > > is working as soon as possible in order to fail early if this is not the
> > > case.  
> > 
> > The read helper seems like a log of general boiler plate code.
> > I briefly looked into the use of regmap for the spi communication,
> > but it did not look like it supported the bit9 stuff.
> > 
> > I did not stare enough to add a reviewd by, but the approach is fine
> > and it is good to detech issues early.  
> 
> The st7789v datasheet describes a setup where SPI is connected
> unidirectional (i.e. without MISO). In that case the ID check
> will fail.

Right. I'll add a (spi->mode & SPI_NO_RX) check, as the default is to
have both lines, if there is no MISO line, I'd expect it to be
described in the DT, otherwise the description is broken.

Thanks,
Miquèl