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Message-ID: <c70935815ffc29ae5256b94c0e4880952abad79c.camel@ndufresne.ca>
Subject: Re: [RFC PATCH v2 2/2] media: rkvdec: rewrite parts of the driver
 in Rust
From: Nicolas Dufresne <nicolas@ndufresne.ca>
To: Daniel Almeida <daniel.almeida@collabora.com>, wedsonaf@gmail.com, 
	ojeda@kernel.org, mchehab@kernel.org, hverkuil@xs4all.nl
Cc: rust-for-linux@vger.kernel.org, linux-kernel@vger.kernel.org, 
	linux-media@vger.kernel.org, kernel@collabora.com
Date: Thu, 07 Mar 2024 15:56:03 -0500
In-Reply-To: <20240307190841.10260-3-daniel.almeida@collabora.com>
References: <20240227215146.46487-1-daniel.almeida@collabora.com>
	 <20240307190841.10260-1-daniel.almeida@collabora.com>
	 <20240307190841.10260-3-daniel.almeida@collabora.com>
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Hi Daniel,

as I'm already sold to the idea, I decided to discuss other things ;-P see
below.

Le jeudi 07 mars 2024 =C3=A0 16:08 -0300, Daniel Almeida a =C3=A9crit=C2=A0=
:
> +++ b/drivers/staging/media/rkvdec/regs.rs
> @@ -0,0 +1,237 @@
> +#![allow(dead_code)]
> +#![allow(unused_macros)]
> +
> +pub(crate) const RKVDEC_REG_INTERRUPT: u32 =3D 0x004;
> +pub(crate) const RKVDEC_INTERRUPT_DEC_E: u32 =3D 1 << 0;
> +pub(crate) const RKVDEC_CONFIG_DEC_CLK_GATE_E: u32 =3D 1 << 1;
> +pub(crate) const RKVDEC_E_STRMD_CLKGATE_DIS: u32 =3D 1 << 2;

Regs file are a bit our coded reference information on how the registers ar=
e
layout in memory. So I believe the alignment, indent and readability of tha=
t
file would at least need polishing.

But to the light of your comment, being able to use more modern utility, is=
n't
there something in Rust we could use to better map the registers ? These
variables are just mask offset to help someone write specific bits within a=
 list
of 32bit registers (Hantro and RKVDEC have that in common). In downstream m=
pp
userspace driver, they maps all the register with a C struct.

struct reg123 {
  val1 :3  // bit 31-29
  val2 :20 // bit 28-9
  val3 :9  // bit 8-0
};

I seriously think it looks nicer, and when the compiler does not screw it u=
p
(the main reason we don't use that), it is also a lot safer and simpler to =
use.
Now, lets forget about C, my question is just if there is something in Rust=
 that
could give us the safety to edit the right portion of a register, but also =
allow
expressing that map in a readable form.

Note that sometimes, we may want to read the register before editing it,
something MPP giant C struct does not help with.

Nicolas