linux-kernel - Re: [PATCH 5.10 04/57] net: octeontx2: Make sure the buffer is 128 byte aligned

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Date:   Sun, 7 Feb 2021 18:39:36 +0800
From:   Kevin Hao <haokexin@...il.com>
To:     Pavel Machek <pavel@....cz>
Cc:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        linux-kernel@...r.kernel.org, stable@...r.kernel.org,
        Subbaraya Sundeep <sbhatta@...vell.com>,
        Jakub Kicinski <kuba@...nel.org>
Subject: Re: [PATCH 5.10 04/57] net: octeontx2: Make sure the buffer is 128
 byte aligned

On Sun, Feb 07, 2021 at 10:20:15AM +0100, Pavel Machek wrote:
> Hi!
> 
> > commit db2805150a0f27c00ad286a29109397a7723adad upstream.
> > 
> > The octeontx2 hardware needs the buffer to be 128 byte aligned.
> > But in the current implementation of napi_alloc_frag(), it can't
> > guarantee the return address is 128 byte aligned even the request size
> > is a multiple of 128 bytes, so we have to request an extra 128 bytes and
> > use the PTR_ALIGN() to make sure that the buffer is aligned correctly.
> > 
> > +++ b/drivers/net/ethernet/marvell/octeontx2/nic/otx2_common.c
> > @@ -473,10 +473,11 @@ dma_addr_t __otx2_alloc_rbuf(struct otx2
> >  	dma_addr_t iova;
> >  	u8 *buf;
> >  
> > -	buf = napi_alloc_frag(pool->rbsize);
> > +	buf = napi_alloc_frag(pool->rbsize + OTX2_ALIGN);
> >  	if (unlikely(!buf))
> >  		return -ENOMEM;
> >  
> > +	buf = PTR_ALIGN(buf, OTX2_ALIGN);
> 
> So we allocate a buffer, then change it, and then pass modified
> pointer to the page_frag_free(buf); in the error path. That... can't
> be right, right?

It doesn't matter. It will work as far as the address we passed to page_frag_free()
is in the range of buf ~ (buf + rbsize).

> 
> >  	iova = dma_map_single_attrs(pfvf->dev, buf, pool->rbsize,
> >  				    DMA_FROM_DEVICE, DMA_ATTR_SKIP_CPU_SYNC);
> >  	if (unlikely(dma_mapping_error(pfvf->dev, iova))) {
> 
> BTW otx2_alloc_rbuf and __otx2_alloc_rbuf should probably return s64,
> as they return negative error code...

It does seem buggy to return dma_addr_t for these two functions, I will cook up
a patch to fix this issue.

Thanks,
Kevin

> 
> Best regards,
> 							Pavel
> 
> -- 
> http://www.livejournal.com/~pavelmachek



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