netdev - RE: [PATCH] rtw88: pci: Use general byte arrays as the elements of RX ring

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Date:   Thu, 25 Jul 2019 09:21:08 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Jian-Hong Pan' <jian-hong@...lessm.com>,
        Yan-Hsuan Chuang <yhchuang@...ltek.com>,
        Kalle Valo <kvalo@...eaurora.org>,
        "David S . Miller" <davem@...emloft.net>
CC:     "linux-wireless@...r.kernel.org" <linux-wireless@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux@...lessm.com" <linux@...lessm.com>,
        "stable@...r.kernel.org" <stable@...r.kernel.org>
Subject: RE: [PATCH] rtw88: pci: Use general byte arrays as the elements of RX
 ring

From: Jian-Hong Pan
> Sent: 25 July 2019 09:09
> Each skb as the element in RX ring was expected with sized buffer 8216
> (RTK_PCI_RX_BUF_SIZE) bytes. However, the skb buffer's true size is
> 16640 bytes for alignment after allocated, x86_64 for example. And, the
> difference will be enlarged 512 times (RTK_MAX_RX_DESC_NUM).
> To prevent that much wasted memory, this patch follows David's
> suggestion [1] and uses general buffer arrays, instead of skbs as the
> elements in RX ring.
...
>  	for (i = 0; i < len; i++) {
> -		skb = dev_alloc_skb(buf_sz);
> -		if (!skb) {
> +		buf = devm_kzalloc(rtwdev->dev, buf_sz, GFP_ATOMIC);

You should do this allocation somewhere than can sleep.
So you don't need GFP_ATOMIC, making the allocate (and dma map)
much less likely to fail.
If they do fail using a smaller ring might be better than failing
completely.

I suspect that buf_sz gets rounded up somewhat.
Also you almost certainly want 'buf' to be cache-line aligned.
I don't think devm_kzalloc() guarantees that at all.

While allocating all 512 buffers in one block (just over 4MB)
is probably not a good idea, you may need to allocated (and dma map)
then in groups.

	David

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