linux-kernel - Re: [PATCH 13/16] gpu: nova-core: Add support for VBIOS ucode extraction for boot

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Message-ID: <aAkBIvfTkKVNbdnm@pollux>
Date: Wed, 23 Apr 2025 17:02:58 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Joel Fernandes <joelagnelf@...dia.com>
Cc: Alexandre Courbot <acourbot@...dia.com>,
	Miguel Ojeda <ojeda@...nel.org>,
	Alex Gaynor <alex.gaynor@...il.com>,
	Boqun Feng <boqun.feng@...il.com>, Gary Guo <gary@...yguo.net>,
	Björn Roy Baron <bjorn3_gh@...tonmail.com>,
	Benno Lossin <benno.lossin@...ton.me>,
	Andreas Hindborg <a.hindborg@...nel.org>,
	Alice Ryhl <aliceryhl@...gle.com>, Trevor Gross <tmgross@...ch.edu>,
	David Airlie <airlied@...il.com>, Simona Vetter <simona@...ll.ch>,
	Maarten Lankhorst <maarten.lankhorst@...ux.intel.com>,
	Maxime Ripard <mripard@...nel.org>,
	Thomas Zimmermann <tzimmermann@...e.de>,
	Jonathan Corbet <corbet@....net>,
	John Hubbard <jhubbard@...dia.com>, Ben Skeggs <bskeggs@...dia.com>,
	Timur Tabi <ttabi@...dia.com>, Alistair Popple <apopple@...dia.com>,
	linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org,
	nouveau@...ts.freedesktop.org, dri-devel@...ts.freedesktop.org
Subject: Re: [PATCH 13/16] gpu: nova-core: Add support for VBIOS ucode
 extraction for boot

On Wed, Apr 23, 2025 at 10:52:42AM -0400, Joel Fernandes wrote:
> Hello, Danilo,
> Thanks for all the feedback. Due to the volume of feedback, I will respond
> incrementally in multiple emails so we can discuss as we go - hope that's Ok but
> let me know if that is annoying.

That's perfectly fine, whatever works best for you. :)

> On 4/23/2025 10:06 AM, Danilo Krummrich wrote:
> 
> >> +impl Vbios {
> >> +    /// Read bytes from the ROM at the current end of the data vector
> >> +    fn read_more(bar0: &Devres<Bar0>, data: &mut KVec<u8>, len: usize) -> Result {
> >> +        let current_len = data.len();
> >> +        let start = ROM_OFFSET + current_len;
> >> +
> >> +        // Ensure length is a multiple of 4 for 32-bit reads
> >> +        if len % core::mem::size_of::<u32>() != 0 {
> >> +            pr_err!("VBIOS read length {} is not a multiple of 4\n", len);
> > 
> > Please don't use any of the pr_*() print macros within a driver, use the dev_*()
> > ones instead.
> 
> Ok I'll switch to this. One slight complication is I've to retrieve the 'dev'
> from the Bar0 and pass that along, but that should be doable.

You can also pass the pci::Device reference to VBios::probe() directly.

> 
> > 
> >> +            return Err(EINVAL);
> >> +        }
> >> +
> >> +        // Allocate and zero-initialize the required memory
> > 
> > That's obvious from the code, if you feel this needs a comment, better explain
> > what we need it for, why zero-initialize, etc.
> 
> Sure, actually the extends_with() is a performance optimization as we want to do
> only a single allocation and then fill in the allocated data. It has nothing to
> do with 0-initializing per-se. I will adjust the comment, but:
> 
> This code...
> 
> >> +        data.extend_with(len, 0, GFP_KERNEL)?;
> >> +        with_bar!(?bar0, |bar0_ref| {
> >> +            let dst = &mut data[current_len..current_len + len];
> >> +            for (idx, chunk) in dst
> >> +                .chunks_exact_mut(core::mem::size_of::<u32>())
> >> +                .enumerate()
> >> +            {
> >> +                let addr = start + (idx * core::mem::size_of::<u32>());
> >> +                // Convert the u32 to a 4 byte array. We use the .to_ne_bytes()
> >> +                // method out of convenience to convert the 32-bit integer as it
> >> +                // is in memory into a byte array without any endianness
> >> +                // conversion or byte-swapping.
> >> +                chunk.copy_from_slice(&bar0_ref.try_read32(addr)?.to_ne_bytes());
> >> +            }
> >> +            Ok(())
> >> +        })?;
> >> +
> >> +        Ok(())
> >> +    }
> ..actually initially was:
> 
> +        with_bar!(self.bar0, |bar0| {
> +            // Get current length
> +            let current_len = self.data.len();
> +
> +            // Read ROM data bytes push directly to vector
> +            for i in 0..bytes as usize {
> +                // Read byte from the VBIOS ROM and push it to the data vector
> +                let rom_addr = ROM_OFFSET + current_len + i;
> +                let byte = bar0.try_readb(rom_addr)?;
> +                self.data.push(byte, GFP_KERNEL)?;
> 
> Where this bit could result in a lot of allocation.
> 
> There was an unsafe() way of not having to do this but we settled with
> extends_with().
> 
> Thoughts?

If I understand you correctly, you just want to make sure that subsequent push()
calls don't re-allocate? If so, you can just use reserve() [1] and keep the
subsequent push() calls.

[1] https://rust.docs.kernel.org/kernel/alloc/kvec/struct.Vec.html#method.reserve