linux-kernel - Re: [PATCH net-next v2 2/2] net: mvneta: Use cacheable memory to store the rx buffer DMA address

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Date:   Fri, 17 Feb 2017 16:20:34 +0100
From:   Gregory CLEMENT <gregory.clement@...e-electrons.com>
To:     Thomas Petazzoni <thomas.petazzoni@...e-electrons.com>
Cc:     Jisheng Zhang <jszhang@...vell.com>, <davem@...emloft.net>,
        <arnd@...db.de>, netdev@...r.kernel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH net-next v2 2/2] net: mvneta: Use cacheable memory to store the rx buffer DMA address

Hi Thomas,
 
 On ven., févr. 17 2017, Thomas Petazzoni <thomas.petazzoni@...e-electrons.com> wrote:

> Does not make sense, because it's not the SW that refills the RX
> descriptors with the address of the RX buffers. It's done by the HW.
>
> With HWBM, I believe you have no choice but to read the physical
> address from the RX descriptor. But you can probably optimize things a
> little bit by reading it only once, and then storing it into a
> cacheable variable.
>
> So maybe:
>
>  - For SWBM, use the strategy proposed by Jisheng
>  - For HWBM, at the beginning of the RX completion path, read once the
>    rx_desc->buf_phys_addr, and store it in rxq->buf_dma_addr[index]


For the HWBM path storing rx_desc->buf_phys_addr in
rxq->buf_dma_addr[index] is not useful as we only use it in a single
function.

But a quick improvement could be to use the phys_addr variable. Indeed
we store the value of rx_desc->buf_phys_addr in it and we never used it,
instead we always use rx_desc->buf_phys_addr.

Gregory

>
> Of course that's just a very rough proposal. I've been looking mainly
> at mvpp2 lately, and I'm not sure I still remember how mvneta works in
> the details.
>
> Best regards,
>
> Thomas
> -- 
> Thomas Petazzoni, CTO, Free Electrons
> Embedded Linux and Kernel engineering
> http://free-electrons.com

-- 
Gregory Clement, Free Electrons
Kernel, drivers, real-time and embedded Linux
development, consulting, training and support.
http://free-electrons.com