Re: [PATCH v5 1/2] drm/panel: Add support for Truly NT35597 panel

[Thierry Reding <thierry.reding@gmail.com>]

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On Fri, Aug 03, 2018 at 01:03:45PM +0200, Linus Walleij wrote:
> On Fri, Aug 3, 2018 at 4:49 AM Abhinav Kumar <abhinavk@codeaurora.org> wrote:
> 
> Hi Abhinav,
> 
> > From: "abhinavk@codeaurora.org" <abhinavk@codeaurora.org>
> >
> > Add support for Truly NT35597 panel used
> > in MSM reference platforms.
> >
> > This panel supports both single DSI and dual DSI
> > modes.
> >
> > However, this patch series adds support only for
> > dual DSI mode.
> >
> > Changes in v5:
> > - Added comments for the delays
> > - Fix error messages and return code
> > - Start using backlight_enable/disable helpers
> > - Start using ARRAY_SIZE everywhere
> > - Split the panel commands into three sets to
> >   remove redundant structure fields and simplify
> >   the DCS command sending method
> > - Use of_get_drm_display_mode() and simplify
> >   get_modes function
> > - Remove truly_wqxga_panel_del and do necessary
> >   cleanup
> > - Replace dev_err with DRM_DEV_ERROR
> >
> > Signed-off-by: Archit Taneja <architt@codeaurora.org>
> > Signed-off-by: Abhinav Kumar <abhinavk@codeaurora.org>
> 
> Overall this driver looks good to me.
> 
> Just a question:
> 
> > +struct cmd_set panel_cmds_set_1[] = {
> > +       /* CMD2_P0 */
> > +       { { 0xff, 0x20 } },
> > +       { { 0xfb, 0x01 } },
> > +       { { 0x00, 0x01 } },
> 
> This is what we call a jam table, I guess "magic init sequence".
> 
> There are some comments on what the different sections do, but in
> cases like this where there is no public datasheet, it would be nice
> to use some #defines rather than opaque hex codes, if you know what
> the different commands actually mean.
> 
> This is in order to help others with hacking on the driver.
> 
> If you don't have more info than this it's fine, just asking.
> 
> > +       /* Resolution:1440x2560 */
> > +       { { 0x72, 0x02 } },
> 
> This is for example quite hard-coded. One gets the idea that the
> resolution is dynamic and that this is not really a panel per se but
> a panel driver, so the Truly NT35597 is not a panel but a panel driver
> that can be configured to be used with several physical panels.
> 
> Compare to other panel drivers such as Ilitek ILI9322 that is in this
> driver dir. There I make it a bit more transparent what the panel driver
> is actually doing on the inside, so if we find it is used with other
> physical panels we can reuse the code more easily.
> 
> > +       truly_write_buf_func(ret, truly_dcs_write_buf,
> > +               panel, SHORT_PACKET,
> > +               ARRAY_SIZE(panel_cmds_set_1),
> > +               panel_cmds_set_1);
> 
> Instead of calling these "cmd_set_1" name them after what the
> command set actually does so we can follow the init flow.
> If you don't know what the commands do you could as well
> call it "magic 1", "magic 2" etc so we know it is magic.
> 
> > +static const struct of_device_id truly_wqxga_of_match[] = {
> > +       { .compatible = "truly,nt35597", },
> 
> If this is a panel driver that not only configurable for wqxga but actually
> also other resolutions this is misleading.
> 
> I suspect this is indeed a panel driver and not a panel with integrated
> driver. I think the best is to define two compatible strings like
> we do for ILI9322:
> "truly,nt35597", "qcom,reference-design-name-display";

I don't understand why we need the two compatible strings for this.
Having "truly,nt35597" isn't quite correct in that case, because in
itself that doesn't contain enough information for any programming.

If that chip can indeed be used to drive different panels, what we
really need is the a compatible string that describes the complete
assembly. In the driver we could then rely on parameterized common
code that the panel driver can call into in order to program the
driver chip.

Note that a driver is assumed to know what to do with a device that
it gets bound to based on the compatible string. If some OS implements a
driver for "truly,nt35597" but not "qcom,reference-design-name-display",
then what is it supposed to do if it encounters the above list of
compatible strings? It can't really program a mode because its missing
essential information such as resolution or timings.

Thierry

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