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From: Kalle Valo <>
To: Christian Lamparter <>
Cc: Jia-Ju Bai <>,,
Subject: Re: [PATCH] p54: avoid accessing the data mapped to streaming DMA
Date: Wed, 26 Aug 2020 19:02:43 +0300	[thread overview]
Message-ID: <> (raw)
In-Reply-To: <> (Christian Lamparter's message of "Tue, 18 Aug 2020 19:27:10 +0200")

Christian Lamparter <> writes:

> On 2020-08-02 15:29, Jia-Ju Bai wrote:
>> In p54p_tx(), skb->data is mapped to streaming DMA on line 337:
>>    mapping = pci_map_single(..., skb->data, ...);
>> Then skb->data is accessed on line 349:
>>    desc->device_addr = ((struct p54_hdr *)skb->data)->req_id;
>> This access may cause data inconsistency between CPU cache and hardware.
>> To fix this problem, ((struct p54_hdr *)skb->data)->req_id is stored in
>> a local variable before DMA mapping, and then the driver accesses this
>> local variable instead of skb->data.
> Interesting. Please bear with me here. From my understanding, the
> streaming direction is set to PCI_DMA_TODEVICE. So is it really
> possible for the hardware to interfere with the data without the IOMMU
> catching this?

Also is there any documentation about this scenario? I would like to
understand this better.

> (That said, patch looks be fine. I'll need to dust off a old PCI PC to
> check this with real hardware, if requested.)

No need to test with real hardware for my sake, but a careful review is
very much appreciated.


  reply	other threads:[~2020-08-26 16:03 UTC|newest]

Thread overview: 7+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-08-02 13:29 [PATCH] p54: avoid accessing the data mapped to streaming DMA Jia-Ju Bai
2020-08-18 12:41 ` Kalle Valo
2020-08-18 17:27 ` Christian Lamparter
2020-08-26 16:02   ` Kalle Valo [this message]
2020-08-28 21:19     ` Christian Lamparter
2020-08-31 14:00       ` Kalle Valo
2020-09-01  9:34 ` Kalle Valo

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