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Re: [PATCH] hw/net: fsl_etsec: Tx padding length should exclude CRC
From: |
David Gibson |
Subject: |
Re: [PATCH] hw/net: fsl_etsec: Tx padding length should exclude CRC |
Date: |
Tue, 23 Mar 2021 11:08:13 +1100 |
On Mon, Mar 22, 2021 at 01:48:07PM +0800, Bin Meng wrote:
> Hi David,
>
> On Mon, Mar 22, 2021 at 1:24 PM David Gibson
> <david@gibson.dropbear.id.au> wrote:
> >
> > On Mon, Mar 22, 2021 at 12:33:06PM +0800, Bin Meng wrote:
> > > Hi David,
> > >
> > > On Mon, Mar 22, 2021 at 12:11 PM David Gibson
> > > <david@gibson.dropbear.id.au> wrote:
> > > >
> > > > On Tue, Mar 16, 2021 at 04:15:05PM +0800, Bin Meng wrote:
> > > > > As the comment of tx_padding_and_crc() says: "Never add CRC in QEMU",
> > > > > min_frame_len should excluce CRC, so it should be 60 instead of 64.
> > > >
> > > > Sorry, your reasoning still isn't clear to me. If qemu is not adding
> > > > the CRC, what is?
> > >
> > > No one is padding CRC in QEMU. QEMU network backends pass payload
> > > without CRC in between.
> >
> > Ok, but the CRCs must be added if the packets are bridged onto a real
> > device, yes? Where does that happen?
>
> I've never used it like that before. What's the command line to test that?
>
> > >
> > > > Will it always append a CRC after this padding is complete?
> > >
> > > No.
> >
> > If that's true, then won't the packets still be shorter than expected
> > if we only pad to 60 bytes?
>
> In QEMU packets are transmitted without CRC between network backends,
> and when a NIC receives a packet, the minimum required payload length
> is 60 bytes without a CRC.
Ok, I see what you're saying, and indeed it already pads to 60 bytes,
rather than 64 on the Rx side.
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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Re: [PATCH] hw/net: fsl_etsec: Tx padding length should exclude CRC, David Gibson, 2021/03/22