Re: [PATCH 1/2] net: mv643xx_eth: use {readl|writel}_relaxed instead of readl/writel

Jisheng Zhang
2016-05-13T12:24:19Z

Dear Arnd,

On Fri, 13 May 2016 14:09:43 +0200 Arnd Bergmann wrote:

> On Friday 13 May 2016 19:59:19 Jisheng Zhang wrote:
> > /* port register accessors **************************************************/
> > static inline u32 rdl(struct mv643xx_eth_private *mp, int offset)
> > {
> > - return readl(mp->shared->base + offset);
> > + return readl_relaxed(mp->shared->base + offset);
> > }
> >
> > static inline u32 rdlp(struct mv643xx_eth_private *mp, int offset)
> > {
> > - return readl(mp->base + offset);
> > + return readl_relaxed(mp->base + offset);
> > }
>
> I'd recommend not changing these in general, but introducing new 'rdl_relaxed()'
> and 'rdlp_relaxed()' etc variants that you use in the code that actually
> is performance sensitive, but use the normal non-relaxed versions by
> default.
>
> Then add a comment to each use of the relaxed accessors on how you know
> that it's safe for that caller. This usually is just needed for the xmit()
> function and for the interrupt handler.

Got your points and I do think it makes sense. But could we always use the
relaxed version to save some LoCs, although I have no mv643xx_eth platform
but I can confirm similar relaxed version changes in pxa168_eth is safe and
this is what we do in product's kernel.

Above is just my humble opinion, comments are welcome.

Thanks,
Jisheng

>
> > @@ -2642,10 +2642,10 @@ mv643xx_eth_conf_mbus_windows(struct mv643xx_eth_shared_private *msp,
> > int i;
> >
> > for (i = 0; i < 6; i++) {
> > - writel(0, base + WINDOW_BASE(i));
> > - writel(0, base + WINDOW_SIZE(i));
> > + writel_relaxed(0, base + WINDOW_BASE(i));
> > + writel_relaxed(0, base + WINDOW_SIZE(i));
> > if (i < 4)
> > - writel(0, base + WINDOW_REMAP_HIGH(i));
> > + writel_relaxed(0, base + WINDOW_REMAP_HIGH(i));
> > }
> >
> > win_enable = 0x3f;
>
> Configuring the mbus for instance is clearly not an operation in which
> performance matters at all, so better not touch that.
>
> > @@ -2674,8 +2675,8 @@ static void infer_hw_params(struct mv643xx_eth_shared_private *msp)
> > * [21:8], or a 16-bit coal limit in bits [25,21:7] of the
> > * SDMA config register.
> > */
> > - writel(0x02000000, msp->base + 0x0400 + SDMA_CONFIG);
> > - if (readl(msp->base + 0x0400 + SDMA_CONFIG) & 0x02000000)
> > + writel_relaxed(0x02000000, msp->base + 0x0400 + SDMA_CONFIG);
> > + if (readl_relaxed(msp->base + 0x0400 + SDMA_CONFIG) & 0x02000000)
> > msp->extended_rx_coal_limit = 1;
> > else
> > msp->extended_rx_coal_limit = 0;
>
>
> This also seems to be configuration, rather than in the packet rx/tx hotpath.
>
> Arnd