Re: [PATCH net v4 3/3] net: stmmac: Specify hardware capability value when FIFO size isn't specified

Andrew Lunn
2025-01-31T15:30:20Z

On Fri, Jan 31, 2025 at 03:03:11PM +0000, Steven Price wrote:
> On 31/01/2025 14:47, Andrew Lunn wrote:
> >>> I'm guessing, but in your setup, i assume the value is never written
> >>> to a register, hence 0 is O.K. e.g. dwmac1000_dma_operation_mode_rx(),
> >>> the fifosz value is used to determine if flow control can be used, but
> >>> is otherwise ignored.
> >>
> >> I haven't traced the code, but that fits my assumptions too.
> >
> > I could probably figure it out using code review, but do you know
> > which set of DMA operations your hardware uses? A quick look at
> > dwmac-rk.c i see:
> >
> > /* If the stmmac is not already selected as gmac4,
> > * then make sure we fallback to gmac.
> > */
> > if (!plat_dat->has_gmac4)
> > plat_dat->has_gmac = true;
>
> has_gmac4 is false on this board, so has_gmac will be set to true here.

Thanks. Looking in hwif.c, that means dwmac1000_dma_ops are used.

dwmac1000_dma_operation_mode_rx() just does a check:

if (rxfifosz < 4096) {
csr6 &= ~DMA_CONTROL_EFC;

but otherwise does not use the value.

dwmac1000_dma_operation_mode_tx() appears to completely ignore fifosz.

So i would say all zero is valid for has_gmac == true, but you might
gain flow control if a value was passed.

A quick look at dwmac100_dma_operation_mode_tx() suggests fifosz is
also ignored, and dwmac100_dma_operation_mode_rx() does not exist. So
all 0 is also valid for gmac == false, gmac4 == false, and xgmac ==
false.

dwmac4_dma_rx_chan_op_mode() does use the value to determine mtl_rx_op
which gets written to a register. So gmac4 == true looks to need
values. dwxgmac2_dma_rx_mode() is the same, so xgmac = true also need
valid values.

Could you cook up a fix based on this?

Andrew