Re: [PATCH net-next 2/3] net: page_pool: add the possibility to sync DMA memory for non-coherent devices

Jesper Dangaard Brouer
2019-11-14T21:43:25Z

On Thu, 14 Nov 2019 13:04:26 -0800
"Jonathan Lemon" <jonathan.lemon@gmail.com> wrote:

> On 14 Nov 2019, at 12:42, Ilias Apalodimas wrote:
>
> > Hi Jonathan,
> >
> > On Thu, Nov 14, 2019 at 12:27:40PM -0800, Jonathan Lemon wrote:
> >>
> >>
> >> On 14 Nov 2019, at 10:53, Ilias Apalodimas wrote:
> >>
> >>> [...]
> >>>>> index 2cbcdbdec254..defbfd90ab46 100644
> >>>>> --- a/include/net/page_pool.h
> >>>>> +++ b/include/net/page_pool.h
> >>>>> @@ -65,6 +65,9 @@ struct page_pool_params {
> >>>>> int nid; /* Numa node id to allocate from pages from */
> >>>>> struct device *dev; /* device, for DMA pre-mapping purposes */
> >>>>> enum dma_data_direction dma_dir; /* DMA mapping direction */
> >>>>> + unsigned int max_len; /* max DMA sync memory size */
> >>>>> + unsigned int offset; /* DMA addr offset */
> >>>>> + u8 sync;
> >>>>> };
> >>>>
> >>>> How about using PP_FLAG_DMA_SYNC instead of another flag word?
> >>>> (then it can also be gated on having DMA_MAP enabled)
> >>>
> >>> You mean instead of the u8?
> >>> As you pointed out on your V2 comment of the mail, some cards don't
> >>> sync back to device.
> >>>
> >>> As the API tries to be generic a u8 was choosen instead of a flag
> >>> to cover these use cases. So in time we'll change the semantics of
> >>> this to 'always sync', 'dont sync if it's an skb-only queue' etc.
> >>>
> >>> The first case Lorenzo covered is sync the required len only instead
> >>> of the full buffer
> >>
> >> Yes, I meant instead of:
> >> + .sync = 1,
> >>
> >> Something like:
> >> .flags = PP_FLAG_DMA_MAP | PP_FLAG_DMA_SYNC
> >>

I actually agree and think we could use a flag. I suggest
PP_FLAG_DMA_SYNC_DEV to indicate that this DMA-sync-for-device.

Ilias notice that the change I requested to Lorenzo, that dma_sync_size
default value is 0xFFFFFFFF (-1). That makes dma_sync_size==0 a valid
value, which you can use in the cases, where you know that nobody have
written into the data-area. This allow us to selectively choose it for
these cases.

--
Best regards,
Jesper Dangaard Brouer
MSc.CS, Principal Kernel Engineer at Red Hat
LinkedIn: http://www.linkedin.com/in/brouer