linux-kernel - Re: [PATCH v4 16/20] nova-core: Add support for VBIOS ucode extraction for boot

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Message-ID: <aEHEIId4Fp0cCWMH@pollux>
Date: Thu, 5 Jun 2025 18:21:52 +0200
From: Danilo Krummrich <dakr@...nel.org>
To: Joel Fernandes <joelagnelf@...dia.com>
Cc: Lyude Paul <lyude@...hat.com>, 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>,
	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,
	Shirish Baskaran <sbaskaran@...dia.com>
Subject: Re: [PATCH v4 16/20] nova-core: Add support for VBIOS ucode
 extraction for boot

On Thu, Jun 05, 2025 at 12:09:46PM -0400, Joel Fernandes wrote:
> >> +impl PmuLookupTable {
> >> +    fn new(pdev: &pci::Device, data: &[u8]) -> Result<Self> {
> >> +        if data.len() < 4 {
> >> +            return Err(EINVAL);
> >> +        }
> >> +
> >> +        let header_len = data[1] as usize;
> >> +        let entry_len = data[2] as usize;
> >> +        let entry_count = data[3] as usize;
> >> +
> >> +        let required_bytes = header_len + (entry_count * entry_len);
> >> +
> >> +        if data.len() < required_bytes {
> >> +            dev_err!(
> >> +                pdev.as_ref(),
> >> +                "PmuLookupTable data length less than required\n"
> >> +            );
> >> +            return Err(EINVAL);
> >> +        }
> >> +
> >> +        // Create a copy of only the table data
> >> +        let table_data = {
> >> +            let mut ret = KVec::new();
> >> +            ret.extend_from_slice(&data[header_len..required_bytes], GFP_KERNEL)?;
> >> +            ret
> >> +        };
> >> +
> >> +        // Debug logging of entries (dumps the table data to dmesg)
> >> +        if cfg!(debug_assertions) {
> >> +            for i in (header_len..required_bytes).step_by(entry_len) {
> >> +                dev_dbg!(
> >> +                    pdev.as_ref(),
> >> +                    "PMU entry: {:02x?}\n",
> >> +                    &data[i..][..entry_len]
> >> +                );
> >> +            }
> >> +        }
> > 
> > Not sure this makes sense - debug_assertions is supposed to be about
> > assertions, we probably shouldn't try to use it for other things (especially
> > since we've already got dev_dbg! here)
> 
> This was suggested by Danilo. I don't really feel strongly either way, IMO I am
> also Ok with running it in production.

When I suggested this, the code looked like this:

	// "last_entry_bytes" is a debugging aid.
	// let mut last_entry_bytes: Option<KVec<u8>> = Some(KVec::new());
	
	for &byte in &data[header_len..required_bytes] {
	    table_data.push(byte, GFP_KERNEL)?;
	    /*
	     * Uncomment for debugging (dumps the table data to dmesg):
	     * last_entry_bytes.as_mut().ok_or(EINVAL)?.push(byte, GFP_KERNEL)?;
	     *
	     * let last_entry_bytes_len = last_entry_bytes.as_ref().ok_or(EINVAL)?.len();
	     * if last_entry_bytes_len == entry_len {
	     *     pr_info!("Last entry bytes: {:02x?}\n", &last_entry_bytes.as_ref().ok_or(EINVAL)?[..]);
	     *     last_entry_bytes = Some(KVec::new());
	     * }
	     */
	}

Now the compiler probably optimizes the loop away, since dev_dbg!() turns into a
noop. So, now we can indeed probably remove it.