netdev - RE: [PATCH] net: fec: check DMA addressing limitations

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Date:   Thu, 2 Aug 2018 07:47:23 +0000
From:   Andy Duan <fugang.duan@....com>
To:     Stefan Agner <stefan@...er.ch>
CC:     "davem@...emloft.net" <davem@...emloft.net>,
        "krzk@...nel.org" <krzk@...nel.org>,
        "robin.murphy@....com" <robin.murphy@....com>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH] net: fec: check DMA addressing limitations

From: Stefan Agner <stefan@...er.ch> Sent: 2018年8月2日 15:16
> On 02.08.2018 04:00, Andy Duan wrote:
> > From: Stefan Agner <stefan@...er.ch> Sent: 2018年8月1日 19:45
> >> Check DMA addressing limitations as suggested by the DMA API how-to.
> >> This does not fix a particular issue seen but is considered good style.
> >>
> >> Signed-off-by: Stefan Agner <stefan@...er.ch>
> >> ---
> >>  drivers/net/ethernet/freescale/fec_main.c | 6 ++++++
> >>  1 file changed, 6 insertions(+)
> >>
> >> diff --git a/drivers/net/ethernet/freescale/fec_main.c
> >> b/drivers/net/ethernet/freescale/fec_main.c
> >> index c729665107f5..af0fb200e936 100644
> >> --- a/drivers/net/ethernet/freescale/fec_main.c
> >> +++ b/drivers/net/ethernet/freescale/fec_main.c
> >> @@ -3146,6 +3146,12 @@ static int fec_enet_init(struct net_device
> >> *ndev)
> >>  	fep->tx_align = 0x3;
> >>  #endif
> >>
> >> +	/* Check mask of the streaming and coherent API */
> >> +	if (dma_set_mask_and_coherent(&fep->pdev->dev,
> >> DMA_BIT_MASK(32))) {
> >> +		dev_warn(&fep->pdev->dev, "No suitable DMA
> available\n");
> >> +		return -ENODEV;
> > It is better:
> >
> > ret = dma_set_mask_and_coherent(&fep->pdev->dev,
> DMA_BIT_MASK(32)); if
> > (ret < 0) {
> > 	dev_warn(&fep->pdev->dev, "No suitable DMA available\n");
> > 	return ret;
> > }
> 
> The code comes from the example in
> Documentation/DMA-API-HOWTO.txt.
> 
> I can rearrange if you prefer.
> 
It is better to return the real error code like -EIO if dma_mask is not initialized. 

> >
> >
> > If the patch aim to "OF: Don't set default coherent DMA mask", I think
> > not only this driver need to add the DMA mask limitations,  many other
> > drivers also need.
> 
> It doesn't exactly address the issue since with that patch DMA mask wasn't
> set at all. This code would just make the driver fail with -ENODEV instead of
> -ENOMEM.
> 
Yes, I agree. It doesn't address the issue if no patch "of/platform: Initialise default DMA masks".

For non-DT, it seems dma_coerce_mask_and_coherent() much better.

> To force a DMA mask I would have to set the DMA mask using
> dma_coerce_mask_and_coherent(). But as discussed with Robin it is safe
> to assume that an initial mask is set by the bus code:
> https://emea01.safelinks.protection.outlook.com/?url=http%3A%2F%2Flk
> ml.kernel.org%2Fr%2F892f9d14-e6fd-7b1b-d07b-af0be6e623fa%40arm.c
> om&amp;data=02%7C01%7Cfugang.duan%40nxp.com%7C618543fb9b4e
> 4f9fa49908d5f847c47f%7C686ea1d3bc2b4c6fa92cd99c5c301635%7C0%
> 7C0%7C636687909487381876&amp;sdata=hyeeh0KSC3Fu7VDl4A8JbcCO
> %2FwocUpTEMvTv1hjba0E%3D&amp;reserved=0
> 
> Setting the DMA mask on DT buses has been addressed by Robin with
> "of/platform: Initialise default DMA masks". So everything should be fine
> again.
> 
Anyway, I am fine with the patch except the return code.
Thanks.

> I agree, other drivers need fixing too.
> 
> --
> Stefan
> 
> >
> >
> >> +	}
> >> +
> >>  	fec_enet_alloc_queue(ndev);
> >>
> >>  	bd_size = (fep->total_tx_ring_size + fep->total_rx_ring_size) *
> >> dsize;
> >> --
> >> 2.18.0