On Mon, Oct 12, 2026 at 12:16 PM <[email protected]> wrote: > Thank you for your contribution! Sashiko AI review found 3 potential > issue(s) to consider: > - [High] drm/vc4: dsi: premature controller enable during timeout reset > causes hardware instability > - [High] drm/vc4: dsi: spurious completion due to IRQ handler TOCTOU race > - [Medium] drm/vc4: missing mutex serialization in DSI host transfer > -- > Thanks for the review. > > 1. Agreed. Mainline's pre_enable ungates the block only after the PHY > timings, timeouts and DISP1 are programmed; the restore must follow > the same order. Will fix in v2. > 2. Agreed. The race predates this patch, but the timeout path is the > right place to close it; v2 will call synchronize_irq() before the > reset. > 3. vc4_dsi_host_transfer() has never serialized transfers. I'll add a > separate patch in v2 that takes a mutex around host transfers. > > Henry >
> commit b73bc40cfc01c81e251315be152ff0c2c6aa9361 > Author: Henry Zhang <[email protected]> > > drm/vc4: dsi: Recover from a timed-out transfer > > When a transfer times out, vc4_dsi_host_transfer() disables the command, > resets the FIFOs and returns -ETIMEDOUT. This commit adds a recovery > mechanism that resets the DSI block and its analog front end after a > timeout and restores the configuration so DSI keeps working without a > modeset. > > diff --git a/drivers/gpu/drm/vc4/vc4_dsi.c b/drivers/gpu/drm/vc4/vc4_dsi.c > index c092a1aaf700a..3783ff3eeac97 100644 > --- a/drivers/gpu/drm/vc4/vc4_dsi.c > +++ b/drivers/gpu/drm/vc4/vc4_dsi.c > [ ... ] > @@ -1359,6 +1358,93 @@ static ssize_t vc4_dsi_host_transfer(struct > mipi_dsi_host *host, > return ret; > } > > +/* > + * Reset the DSI block and its analog front end, then restore the > + * configuration done in pre_enable. Resetting the FIFOs alone does not > + * clear the state the block can be left in after a transfer timed out > + * (e.g. on an LP receive timeout): every following transfer then times > + * out as well, until the next modeset. > + */ > +static void vc4_dsi_reset_and_restore(struct vc4_dsi *dsi) > +{ > + u32 ctrl = DSI_PORT_READ(CTRL); > + u32 afec0 = DSI_PORT_READ(PHY_AFEC0); > [ ... ] > + DSI_PORT_WRITE(DISP0_CTRL, disp0); > + DSI_PORT_WRITE(TXPKT1C, 0); > + DSI_PORT_WRITE(CTRL, DSI_CTRL_SOFT_RESET_CFG | > + DSI_PORT_BIT(CTRL_RESET_FIFOS)); > + DSI_PORT_WRITE(PHY_AFEC0, afec0 | DSI_PORT_BIT(PHY_AFEC0_RESET)); > + /* AFEC reset hold time */ > + mdelay(1); > + > + DSI_PORT_WRITE(CTRL, ctrl & ~DSI_CTRL_CAL_BYTE); > + DSI_PORT_WRITE(HSTX_TO_CNT, hstx_to); > + DSI_PORT_WRITE(LPRX_TO_CNT, lprx_to); > > [Severity: High] > Can writing ctrl & ~DSI_CTRL_CAL_BYTE back to CTRL prematurely un-gate and > enable the controller before the subsequent writes to timing and PHY > configuration registers? > > Since ctrl was read while a transfer was in progress, it contains the > enable > bit (DSI0_CTRL_CTRL0 or DSI1_CTRL_EN). > > Writing it back here enables the hardware block before its timings > (HSTX_TO_CNT, HS_CLT0, PHYC, etc.) are restored, which could cause erratic > bus > behavior and unrecoverable hardware states. > > > + DSI_PORT_WRITE(TA_TO_CNT, ta_to); > [ ... ] > +static ssize_t vc4_dsi_host_transfer(struct mipi_dsi_host *host, > + const struct mipi_dsi_msg *msg) > +{ > + struct vc4_dsi *dsi = host_to_dsi(host); > + u32 disp0_ctrl; > + ssize_t ret; > + > + ret = vc4_dsi_transfer(dsi, msg); > > [Severity: Medium] > Does this entry point need a mutex to serialize concurrent transfers? > > If user-space concurrently triggers DCS commands (e.g., via sysfs backlight > updates and DRM KMS modesets), they can race because vc4_dsi_transfer() > modifies shared variables like dsi->xfer_completion without a lock. > > With this patch, a timeout caused by such a race now triggers a full block > reset in vc4_dsi_reset_and_restore(), disrupting any interleaved concurrent > transfer. > > + if (ret != -ETIMEDOUT) > + return ret; > + > + /* > + * Leave the block usable for the next transfer. The message is not > + * resent: it may already have reached the peripheral, and only the > + * caller knows whether it is safe to repeat. > + */ > + disp0_ctrl = DSI_PORT_READ(DISP0_CTRL); > + dev_warn(&dsi->pdev->dev, > + "transfer timed out (stat 0x%08x), resetting DSI\n", > + DSI_PORT_READ(STAT)); > + vc4_dsi_reset_and_restore(dsi); > + DSI_PORT_WRITE(DISP0_CTRL, disp0_ctrl); > + > + return ret; > +} > > [Severity: High] > Can a timed-out transfer cause a spurious completion of a subsequent > transfer? > > If vc4_dsi_transfer() times out, wait_for_completion_timeout() returns. > However, if the hardware interrupt had fired and the threaded IRQ handler > (vc4_dsi_irq_handler) was preempted before calling > complete(&dsi->xfer_completion), it could still be pending. > > Since there is no synchronization (like synchronize_irq()) in this timeout > recovery path to wait for the IRQ handler to finish, it returns > immediately. > > The caller might retry the transfer, reinitializing dsi->xfer_completion. > The > preempted IRQ thread could then resume, calling complete() and falsely > waking up > the new transfer prematurely while the hardware is still busy. > > -- > Sashiko AI review · > https://sashiko.dev/#/patchset/[email protected]?part=2 >
