Re: [RFC PATCH] hw/dma: don't allow weird transfer lengths for bcm2835

["Alex Bennée" <alex.bennee@linaro.org>]

Yodel Eldar <yodel.eldar@yodel.dev> writes:

(add Florians to CC)

> On 11/11/2025 04:54, Alex Bennée wrote:
>> The datasheet doesn't explicitly say that TXFR_LEN has to be word
>> aligned but the fact there is a DMA_D_WIDTH flag to select between 32
>> bit and 128 bit strongly implies that is how it works. The downstream
>
> At the bottom of page 38, the datasheet [1] states "the DMA can deal
> with byte aligned transfers and will minimise bus traffic by buffering
> and packing misaligned accesses."
>
> IIUC, the *_WIDTH info fields are implied as maxima.
>
> [1] https://datasheets.raspberrypi.com/bcm2835/bcm2835-peripherals.pdf

That reads ambiguously - you could start a misaligned n*WIDTH transfer
and the hardware will write bytes until aligned?

If it does indeed work with byte accesses maybe we can just do:

  if (xlen & 0x3) {
    .. do one byte ..
    xlen -= 1;
  } else {
    .. existing 32 bit code ..
  }

but I guess we need to handle unaligned accesses as well.

Florian,

Can you help clarify what the datasheet means here?

Thanks,

<snip>

>> rpi kernel also goes to efforts to not write sub-4 byte lengths so
>> lets:

-- 
Alex Bennée
Virtualisation Tech Lead @ Linaro