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hw/dma: don't allow weird transfer lengths for bcm2835

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The datasheet doesn't explicitly say that TXFR_LEN has to be word
aligned but the fact there is a DMA_D_WIDTH flag to select between 32
bit and 128 bit strongly implies that is how it works. The downstream
rpi kernel also goes to efforts to not write sub-4 byte lengths so
lets:

  - fail when mis-programmed and report GUEST_ERROR
  - catch setting D_WIDTH for 128 bit and report UNIMP

Yodel did some digging into the specs (see discussion link):

{A} AMBA AXI Protocol Version: 2.0 Specification
    https://documentation-service.arm.com/static/64256e84314e245d086bc88f

{B} BCM2835 ARM Peripherals
    https://datasheets.raspberrypi.com/bcm2835/bcm2835-peripherals.pdf

[1] {A} (p. 10-2)
[2] {B} (p. 51)
[3] {A} (p. 14-5)
[4] {A} (p. 4-3)
[5] {A} (p. 9-4)
[6] {B} (p. 53)

However was unable to come up with an unambiguous conclusion without
testing on the real hardware. So in the absence of certainty and for
the sake of addressing the DoS I suggest we merge as is for now.

Link: https://lore.kernel.org/all/20251111105429.3993300-1-alex.bennee@linaro.org/
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/3201
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-id: 20260710131500.2323848-1-alex.bennee@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
This commit is contained in:
Alex Bennée
2026-07-13 12:34:17 +01:00
committed by Peter Maydell
parent 79cabc6cb7
commit f7f742beb6
1 changed files with 17 additions and 0 deletions
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17
hw/dma/bcm2835_dma.c
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@@ -86,6 +86,23 @@ static void bcm2835_dma_update(BCM2835DMAState *s, unsigned c)
}
xlen_td = xlen;
if (ch->ti & BCM2708_DMA_D_WIDTH) {
qemu_log_mask(LOG_UNIMP, "%s: 128bit transfers not yet supported", __func__);
ch->cs |= BCM2708_DMA_ERR;
break;
}
/*
* Datasheet implies 32bit or 128bit transfers only
*
* TODO: test on real HW and report back.
*/
if (xlen & 0x3) {
qemu_log_mask(LOG_GUEST_ERROR, "%s: bad transfer size\n", __func__);
ch->cs |= BCM2708_DMA_ERR;
break;
}
while (ylen != 0) {
/* Normal transfer mode */
while (xlen != 0) {