[Intel-gfx] [PATCH v9 10/39] drm/i915: Implement HDCP2.2 receiver authentication

[Daniel Vetter]

On Thu, Dec 20, 2018 at 02:28:55PM +0000, Winkler, Tomas wrote:
> 
> 
> > On Wed, 19 Dec 2018, "Winkler, Tomas" <tomas.winkler at intel.com> wrote:
> > >>
> > >> On Wed, 19 Dec 2018, "C, Ramalingam" <ramalingam.c at intel.com> wrote:
> > >> > On 12/19/2018 8:05 PM, Daniel Vetter wrote:
> > >> >> On Thu, Dec 13, 2018 at 09:31:12AM +0530, Ramalingam C wrote:
> > >> >>>   struct intel_hdcp {
> > >> >>> @@ -414,6 +430,24 @@ struct intel_hdcp {
> > >> >>>   	 */
> > >> >>>   	u8 content_type;
> > >> >>>   	struct hdcp_port_data port_data;
> > >> >>> +
> > >> >>> +	u8 is_paired;
> > >> >>> +	u8 is_repeater;
> > >> >> Make these two bool, will simplify the code a bunch.
> > >> >
> > >> > Seems there is a movement for not to use the bool in structures.
> > >>
> > >> No. Please use bools in structs when it makes sense. Avoid bools in
> > >> structs when you need to care about memory footprint or alignment or
> > >> packing or the like. This is not one of those cases.
> > >>
> > >> > Thats why I have changed these from bool to u8 from v8 onwards.
> > >> > Checkpatch also complains on this
> > >>
> > >> Sorry to say, checkpatch is not the authority although we do send out
> > >> automated checkpatch results.
> > >
> > > I believe it was Linus' call to not use  bool in structs at all
> > > https://lkml.org/lkml/2017/11/21/384
> > 
> > I don't care. That's a valid judgement in the context referenced, but the
> > conclusion "no bools in structs at all" isn't. In this case, I think bools are the
> > better option, and anything else makes the code worse.
> 
> The solution was to use bit fields, 
>  unsinged int is_paired:1;
> unsinged int is_repeter:1

This doesn't work with READ_ONCE/WRITE_ONCE, and it generates terrible
assembly (at least gcc is well known for struggling with these, compared
to open-coded bitops). So depending upon what you want to do, and where
youre space/performance tradeoff lies, doing this unconditionally is just
wrong.

It was the right thing for the patch Linus commented on though.
-Daniel

> There is a strong point in consistency so there are no mistakes.
> 
> But frankly I don't really have strong feelings about it.
> 
> Thanks
> Tomas
> 

-- 
Daniel Vetter
Software Engineer, Intel Corporation
http://blog.ffwll.ch