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Message-ID: <d1ec92db-eefe-465a-9eb5-3a57c22b86c3@bp.renesas.com>
Date: Mon, 15 Apr 2024 08:12:06 +0100
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Subject: Re: [PATCH net 2/4] net: ravb: Allow RX loop to move past DMA mapping
 errors
Content-Language: en-GB
To: =?UTF-8?Q?Niklas_S=C3=B6derlund?= <niklas.soderlund+renesas@ragnatech.se>
Cc: Sergey Shtylyov <s.shtylyov@omp.ru>, "David S. Miller"
 <davem@davemloft.net>, Eric Dumazet <edumazet@google.com>,
 Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
 Geert Uytterhoeven <geert+renesas@glider.be>, netdev@vger.kernel.org,
 linux-renesas-soc@vger.kernel.org, linux-kernel@vger.kernel.org
References: <20240411114434.26186-1-paul.barker.ct@bp.renesas.com>
 <20240411114434.26186-3-paul.barker.ct@bp.renesas.com>
 <20240414121719.GB2860391@ragnatech.se>
From: Paul Barker <paul.barker.ct@bp.renesas.com>
Organization: Renesas Electronics Corporation
In-Reply-To: <20240414121719.GB2860391@ragnatech.se>
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From: Paul Barker <paul.barker.ct@bp.renesas.com>
To: =?UTF-8?Q?Niklas_S=C3=B6derlund?= <niklas.soderlund+renesas@ragnatech.se>
Cc: Sergey Shtylyov <s.shtylyov@omp.ru>, "David S. Miller"
 <davem@davemloft.net>, Eric Dumazet <edumazet@google.com>,
 Jakub Kicinski <kuba@kernel.org>, Paolo Abeni <pabeni@redhat.com>,
 Geert Uytterhoeven <geert+renesas@glider.be>, netdev@vger.kernel.org,
 linux-renesas-soc@vger.kernel.org, linux-kernel@vger.kernel.org
Message-ID: <d1ec92db-eefe-465a-9eb5-3a57c22b86c3@bp.renesas.com>
Subject: Re: [PATCH net 2/4] net: ravb: Allow RX loop to move past DMA mapping
 errors
References: <20240411114434.26186-1-paul.barker.ct@bp.renesas.com>
 <20240411114434.26186-3-paul.barker.ct@bp.renesas.com>
 <20240414121719.GB2860391@ragnatech.se>
In-Reply-To: <20240414121719.GB2860391@ragnatech.se>

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On 14/04/2024 13:17, Niklas S=C3=B6derlund wrote:
> Hi Paul,
>=20
> Thanks for your patch.
>=20
> On 2024-04-11 12:44:31 +0100, Paul Barker wrote:
>> The RX loops in ravb_rx_gbeth() and ravb_rx_rcar() skip to the next lo=
op
>> interation if a zero-length descriptor is seen (indicating a DMA mappi=
ng
>> error). However, the current rx descriptor index `priv->cur_rx[q]` was=

>> incremented at the end of the loop and so would not be incremented whe=
n
>> we skip to the next loop iteration. This would cause the loop to keep
>> seeing the same zero-length descriptor instead of moving on to the nex=
t
>> descriptor.
>>
>> As the loop counter `i` still increments, the loop would eventually
>> terminate so there is no risk of being stuck here forever - but we
>> should still fix this to avoid wasting cycles.
>>
>> To fix this, the rx descriptor index is incremented at the top of the
>> loop, in the for statement itself. The assignments of `entry` and `des=
c`
>> are brought into the loop to avoid the need for duplication.
>>
>> Fixes: d8b48911fd24 ("ravb: fix ring memory allocation")
>> Signed-off-by: Paul Barker <paul.barker.ct@bp.renesas.com>
>> ---
>>  drivers/net/ethernet/renesas/ravb_main.c | 25 ++++++++++++-----------=
-
>>  1 file changed, 13 insertions(+), 12 deletions(-)
>>
>> diff --git a/drivers/net/ethernet/renesas/ravb_main.c b/drivers/net/et=
hernet/renesas/ravb_main.c
>> index 70f2900648d4..028ab5c6aaf7 100644
>> --- a/drivers/net/ethernet/renesas/ravb_main.c
>> +++ b/drivers/net/ethernet/renesas/ravb_main.c
>> @@ -775,12 +775,15 @@ static bool ravb_rx_gbeth(struct net_device *nde=
v, int *quota, int q)
>>  	int limit;
>>  	int i;
>> =20
>> -	entry =3D priv->cur_rx[q] % priv->num_rx_ring[q];
>>  	limit =3D priv->dirty_rx[q] + priv->num_rx_ring[q] - priv->cur_rx[q]=
;
>>  	stats =3D &priv->stats[q];
>> =20
>> -	desc =3D &priv->rx_ring[q].desc[entry];
>> -	for (i =3D 0; i < limit && rx_packets < *quota && desc->die_dt !=3D =
DT_FEMPTY; i++) {
>> +	for (i =3D 0; i < limit && rx_packets < *quota; i++, priv->cur_rx[q]=
++) {
>> +		entry =3D priv->cur_rx[q] % priv->num_rx_ring[q];
>> +		desc =3D &priv->rx_ring[q].desc[entry];
>> +		if (desc->die_dt =3D=3D DT_FEMPTY)
>> +			break;
>> +
>>  		/* Descriptor type must be checked before all other reads */
>>  		dma_rmb();
>>  		desc_status =3D desc->msc;
>> @@ -848,9 +851,6 @@ static bool ravb_rx_gbeth(struct net_device *ndev,=
 int *quota, int q)
>>  				break;
>>  			}
>>  		}
>> -
>> -		entry =3D (++priv->cur_rx[q]) % priv->num_rx_ring[q];
>> -		desc =3D &priv->rx_ring[q].desc[entry];
>>  	}
>> =20
>>  	/* Refill the RX ring buffers. */
>> @@ -891,7 +891,6 @@ static bool ravb_rx_rcar(struct net_device *ndev, =
int *quota, int q)
>>  {
>>  	struct ravb_private *priv =3D netdev_priv(ndev);
>>  	const struct ravb_hw_info *info =3D priv->info;
>> -	int entry =3D priv->cur_rx[q] % priv->num_rx_ring[q];
>>  	struct net_device_stats *stats =3D &priv->stats[q];
>>  	struct ravb_ex_rx_desc *desc;
>>  	struct sk_buff *skb;
>> @@ -900,12 +899,17 @@ static bool ravb_rx_rcar(struct net_device *ndev=
, int *quota, int q)
>>  	int rx_packets =3D 0;
>>  	u8  desc_status;
>>  	u16 pkt_len;
>> +	int entry;
>>  	int limit;
>>  	int i;
>> =20
>>  	limit =3D priv->dirty_rx[q] + priv->num_rx_ring[q] - priv->cur_rx[q]=
;
>> -	desc =3D &priv->rx_ring[q].ex_desc[entry];
>> -	for (i =3D 0; i < limit && rx_packets < *quota && desc->die_dt !=3D =
DT_FEMPTY; i++) {
>> +	for (i =3D 0; i < limit && rx_packets < *quota; i++, priv->cur_rx[q]=
++) {
>> +		entry =3D priv->cur_rx[q] % priv->num_rx_ring[q];
>> +		desc =3D &priv->rx_ring[q].ex_desc[entry];
>> +		if (desc->die_dt =3D=3D DT_FEMPTY)
>> +			break;
>=20
> I really like moving the assignment of entry and desc to the top of the=
=20
> loop. But I don't like the loop limits as it's hard, at least for me, t=
o=20
> immediately see what's going on. How about,
>=20
>     limit =3D min(priv->dirty_rx[q] + priv->num_rx_ring[q] - priv->cur_=
rx[q], *quota);
>=20
>     for (i =3D 0; i < limit; i++) {
>         entry =3D priv->cur_rx[q] % priv->num_rx_ring[q];
>         desc =3D &priv->rx_ring[q].ex_desc[entry];
>=20
>         /* There are no more valid descriptors after an empty one. */
>         if (desc->die_dt =3D=3D DT_FEMPTY)
>                 break;
>=20
>         ...
>     }

We need to count received packets separately from the number of
descriptors processed, as done in the previous commit in this series,
so we can't just have a single check against limit.

We also need to increment priv->cur_rx[q]. If we put `priv->cur_rx[q]++`
at the end of the loop then we're back to having to worry about it when
we have a continue statement.

We could move the `rx_packets < *quota` check inside the loop itself,
but I don't see that as any clearer myself.

Thanks,

--=20
Paul Barker
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