Re: [PATCH 2/7] gpu: nova-core: firmware: riscv: use dma::Coherent

["Gary Guo" <gary@garyguo.net>]

On Mon Mar 23, 2026 at 6:15 AM GMT, Alexandre Courbot wrote:
> On Sat Mar 21, 2026 at 11:58 PM JST, Gary Guo wrote:
>> On Sat Mar 21, 2026 at 1:36 PM GMT, Alexandre Courbot wrote:
>>> Replace the nova-core local `DmaObject` with a `Coherent` that can
>>> fulfill the same role.
>>>
>>> Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
>>> ---
>>>  drivers/gpu/nova-core/firmware/riscv.rs | 6 +++---
>>>  1 file changed, 3 insertions(+), 3 deletions(-)
>>>
>>> diff --git a/drivers/gpu/nova-core/firmware/riscv.rs b/drivers/gpu/nova-core/firmware/riscv.rs
>>> index 14aad2f0ee8a..2afa7f36404e 100644
>>> --- a/drivers/gpu/nova-core/firmware/riscv.rs
>>> +++ b/drivers/gpu/nova-core/firmware/riscv.rs
>>> @@ -5,13 +5,13 @@
>>>  
>>>  use kernel::{
>>>      device,
>>> +    dma::Coherent,
>>>      firmware::Firmware,
>>>      prelude::*,
>>>      transmute::FromBytes, //
>>>  };
>>>  
>>>  use crate::{
>>> -    dma::DmaObject,
>>>      firmware::BinFirmware,
>>>      num::FromSafeCast, //
>>>  };
>>> @@ -66,7 +66,7 @@ pub(crate) struct RiscvFirmware {
>>>      /// Application version.
>>>      pub(crate) app_version: u32,
>>>      /// Device-mapped firmware image.
>>> -    pub(crate) ucode: DmaObject,
>>> +    pub(crate) ucode: Coherent<[u8]>,
>>>  }
>>>  
>>>  impl RiscvFirmware {
>>> @@ -81,7 +81,7 @@ pub(crate) fn new(dev: &device::Device<device::Bound>, fw: &Firmware) -> Result<
>>>              let len = usize::from_safe_cast(bin_fw.hdr.data_size);
>>>              let end = start.checked_add(len).ok_or(EINVAL)?;
>>>  
>>> -            DmaObject::from_data(dev, fw.data().get(start..end).ok_or(EINVAL)?)?
>>> +            Coherent::from_slice(dev, fw.data().get(start..end).ok_or(EINVAL)?, GFP_KERNEL)?
>>
>> `DmaObject` rounds the data up to be page-sized, while this new API doesn't.
>>
>> It has impact on alignment, as the allocator aligns things to the largest
>> power-of-two exponent of the allocated size.
>
> Doesn't `dma_alloc_coherent` always allocate from the page pool and thus
> returns page-aligned memory though?

Oh you're right, this is not from DMA pool, so allocations are page-aligned
indeed.

I brought this up because I got bite by this size adjustment behaviour
because in the `from_data` I initially put

    unsafe { dma_obj.as_mut().copy_from_slice(data) };

but that doesn't work due to the size being aligned up so dma_obj.len() !=
data.len().

But if this behaviour is not needed it does simplify things quite a bit.

> I'm not sure why `DmaObject` was doing that but it was redundant I think.

A question for yourself I guess? :-)

https://lore.kernel.org/all/20250619-nova-frts-v6-13-ecf41ef99252@nvidia.com/

Best,
Gary