[PATCH] drm/bridge/sii902x: Fix EDID readback

[Fabrizio Castro]

> > Here (and also in sii902x_i2c_bypass_deselect) you do a rmw access to the
> > SII902X_SYS_CTRL_DATA register without coordinating with regmap. Regmap is
> > also doing rmw accesses to that register in other parts of the driver. I
> > think you need to either add comment as to why that is safe (maybe other
> > things make it impossible for the two rmw accesses to cross?), or add the
> > missing coordination.
> >
>
> The other two places where SII902X_SYS_CTRL_DATA is being handled are
> sii902x_bridge_disable and sii902x_bridge_enable, I didn’t think there is
> any chance of the modes being probed while the bridge gets enabled/disabled,
> but now that you make me think about it user space may trigger the readback
> of the EDID at the most inconvenient times. Anyway, this is a good point, and
> since I don't think I am supposed to access regmap's lock from outside the APIs,
> I could switch to the unlocked version of update_bits from within sii902x_bridge_disable
> and sii902x_bridge_enable, and manually grab the i2c adapter lock, what do you think?
>

The bridge enable/disable callbacks deal with different bits of register
SII902X_SYS_CTRL_DATA, and the value of register SII902X_SYS_CTRL_DATA  after
sii902x_i2c_bypass_deselect is the same as when sii902x_i2c_bypass_select gets triggered
so basically even if we managed to get in the way of the regmap rmw (in the sense that
for example sii902x_bridge_enable reads SII902X_SYS_CTRL_DATA and then we call
into sii902x_i2c_bypass_select) by the time regmap_update_bits can perform the
write operation the value of register SII902X_SYS_CTRL_DATA is the same as the one
read by regmap, as "select xfer deselect" won't alter the final value of SII902X_SYS_CTRL_DATA
and nobody can get in between.
What do you think I should do? Do you think I can leave this alone and maybe add some
comments or do you think I should explicitly protect access to SII902X_SYS_CTRL_DATA?

Thanks,
Fab





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