Re: [PATCH RFC 2/6] pwm: Add Rust driver for T-HEAD TH1520 SoC

From: Danilo Krummrich <dakr@kernel.org>
To: "Michal Wilczynski/Kernel (PLT) /SRPOL/Engineer/Samsung
	Electronics" <m.wilczynski@samsung.com>
Cc: "Uwe Kleine-König" <ukleinek@kernel.org>,
	"Miguel Ojeda" <ojeda@kernel.org>,
	"Alex Gaynor" <alex.gaynor@gmail.com>,
	"Boqun Feng" <boqun.feng@gmail.com>,
	"Gary Guo" <gary@garyguo.net>,
	"Björn Roy Baron" <bjorn3_gh@protonmail.com>,
	"Benno Lossin" <benno.lossin@proton.me>,
	"Andreas Hindborg" <a.hindborg@kernel.org>,
	"Alice Ryhl" <aliceryhl@google.com>,
	"Trevor Gross" <tmgross@umich.edu>,
	"Drew Fustini" <drew@pdp7.com>, "Guo Ren" <guoren@kernel.org>,
	"Fu Wei" <wefu@redhat.com>, "Rob Herring" <robh@kernel.org>,
	"Krzysztof Kozlowski" <krzk+dt@kernel.org>,
	"Conor Dooley" <conor+dt@kernel.org>,
	"Paul Walmsley" <paul.walmsley@sifive.com>,
	"Palmer Dabbelt" <palmer@dabbelt.com>,
	"Albert Ou" <aou@eecs.berkeley.edu>,
	"Alexandre Ghiti" <alex@ghiti.fr>,
	"Marek Szyprowski" <m.szyprowski@samsung.com>,
	linux-kernel@vger.kernel.org, linux-pwm@vger.kernel.org,
	rust-for-linux@vger.kernel.org, linux-riscv@lists.infradead.org,
	devicetree@vger.kernel.org
Subject: Re: [PATCH RFC 2/6] pwm: Add Rust driver for T-HEAD TH1520 SoC
Date: Tue, 27 May 2025 15:00:27 +0200	[thread overview]
Message-ID: <aDW3a1sjeWWfwaq8@cassiopeiae> (raw)
In-Reply-To: <db8e34c9-daff-43d9-b79b-8ec1bc98a00f@samsung.com>

On Tue, May 27, 2025 at 02:44:57PM +0200, Michal Wilczynski/Kernel (PLT) /SRPOL/Engineer/Samsung Electronics wrote:
> W dniu 25.05.2025 o 14:03, Danilo Krummrich pisze:
> > On Sat, May 24, 2025 at 11:14:56PM +0200, Michal Wilczynski wrote:
> >> diff --git a/drivers/pwm/pwm_th1520.rs b/drivers/pwm/pwm_th1520.rs
> >> new file mode 100644
> >> index 0000000000000000000000000000000000000000..4665e293e8d0bdc1a62a4e295cdaf4d47b3dd134
> >> --- /dev/null
> >> +++ b/drivers/pwm/pwm_th1520.rs
> >> @@ -0,0 +1,272 @@
> >> +// SPDX-License-Identifier: GPL-2.0
> >> +// Copyright (c) 2025 Samsung Electronics Co., Ltd.
> >> +// Author: Michal Wilczynski <m.wilczynski@samsung.com>
> >> +
> >> +//! Rust T-HEAD TH1520 PWM driver
> >> +use kernel::{c_st
> >> +
> >> +struct Th1520PwmChipData {
> >> +    clk: Clk,
> >> +    iomem: kernel::devres::Devres<IoMem<0>>,
> > Why IoMem<0>? If you put the expected memory region size for this chip instead
> > all your subsequent accesses can be iomem.write() / iomem.read() rather than the
> > fallible try_{read,write}() variants.
> The size of the memory region is not known at the compile time. Instead 
> it's configured
> via Device Tree. I'm not sure why it should work differently in Rust ?

There are two sizes:

  (1) The size of the actual MMIO region, which comes from the device-tree.
  (2) The size of the MMIO region that the driver knows it requires to work.

Let's say your driver uses registers with the following offsets.

REG0_OFFSET = 0x0
REG1_OFFSET = 0x4
REG2_OFFSET = 0x100

This means that the size of (2) is 0x100 + width of REG2 (let's say 0x4), which
means that you can safely define your MMIO memory type as IoMem<0x104>.

If you subsequently call pdev.ioremap_resource_sized() it will fail on runtime
if the MMIO region defined in the device-tree does not have a size of at least
0x104, i.e. (1) < (2). That's not a problem because if (1) < (2) your driver
can't work anyways.

In return, you can call the non-try variant of the read / write methods of
IoMem, which do boundary checks on compile time and hence are infallible.

Note that this does not prevent you to still call the try variants of read /
write in case you also have to deal with dynamic offsets that are not known at
compile time.

I hope this helps.

- Danilo