Subject: Re: [EXTERNAL] Re: [PATCH net 1/3] net: ti: icssg-prueth: Use page_pool API for RX buffer allocation

From: Ido Schimmel <[email-masked]>
Date: Wed, 5 Feb 2025 19:41:00 +0200

On Tue, Feb 04, 2025 at 11:25:02PM +0530, Malladi, Meghana wrote:
> Seems like none of the pages which have been allocated aren't getting
> recycled in the rx path after being used unless its some error case. Will
> try to fix this.

skb_mark_for_recycle() should help with page recycling when an skb that
uses them is freed.

Anyway, I believe that I don't see put call when tearing down the Rx
ring because prueth_rx_cleanup() is using page_pool_recycle_direct()
when it shouldn't. AFAICT, prueth_rx_cleanup() is only called from the
control path (upon ndo_stop()) and not in NAPI context.

> Also I have noticed, in prueth_prepare_rx_chan() pages are allocated per
> number of descriptors for a channel, but they are not being used when a
> packet is being recieved (in emac_rx_packet()) and rather new page is
> allocated for the next upcoming packet. Is this a valid design, what are
> your thoughts on this ?

The new page is possibly a page that was recycled into the pool when a
previous packet was freed / dropped.

[...]

> Yes I will add PP_FLAG_DMA_SYNC_DEV as well.
> I believe page_pool_dma_sync_for_cpu() needs to be called sync Rx page for
> CPU, am I right ? If so can you tell me, in what all cases should I call
> this function.

Before accessing the packet data.

> https://lkml.iu.edu/hypermail/linux/kernel/2312.1/06353.html
> In the above link it is quoted - "Note that this version performs DMA sync
> unconditionally, even if the associated PP doesn't perform sync-for-device"
> for the page_pool_dma_sync_for_cpu() function. So does that mean if I am
> using this function I don't need explicily sync for device call?

It's explained in the page pool documentation:

"Driver is always responsible for syncing the pages for the CPU. Drivers
may choose to take care of syncing for the device as well or set the
PP_FLAG_DMA_SYNC_DEV flag to request that pages allocated from the page
pool are already synced for the device."

https://docs.kernel.org/networking/page_pool.html#dma-sync