* Peter Hurley <peter@xxxxxxxxxxxxxxxxxx> [150211 12:05]: > On 02/10/2015 12:46 PM, Peter Hurley wrote: > > On 02/10/2015 07:04 AM, Nicolas Schichan wrote: > >> On 02/10/2015 12:34 AM, Peter Hurley wrote: > >>> Hi Nicolas, > >>> > >>> Thanks for the report. > >>> > >> [...] > >>>> When a caracter is received on the UART while the kernel is printing > >>>> the boot messages, as soon as the kernel configures the UART for > >>>> receiving (after root filesystem mount), it gets stuck printing the > >>>> following message repeatedly: > >>>> > >>>> serial8250: too much work for irq29 > >>>> > >>>> Once stuck, the reception of another character allows the boot process > >>>> to finish. > >>>> > >>>> From what I can gather, when we hit that, the UART_IIR_NO_INT is 0 (so the > >>>> interrupt is raised), but the UART_LSR_DR bit is 0 as well so the UART_RX > >>>> register is never read to clear the interrupt. > >>> > >>> The "too much work" message means serial8250_handle_irq() is returning 0, > >>> ie., not handled. Which in turn means IIR indicates no interrupt is pending > >>> (UART_IIR_NO_INT == 1). > >>> > >>> Can you log the register values for LSR and IIR at both patch locations > >>> in serial8250_do_startup()? > >>> > >>> (I can get you a debug patch, if necessary. Let me know) > >> > >> Hi Peter, > >> > >> Thanks for your reply. > >> > >> Here is what I have when the issue is triggered: > >> > >> [ 12.154877] lsr 0x60 / iir 0x01 > >> [ 12.158071] lsr 0x60 / iir 0x01 > >> [ 12.161438] serial8250: too much work for irq29 > >> [ 12.165982] lsr 0x60 / iir 0x0c > >> [ 12.169354] serial8250: too much work for irq29 > >> [ 12.173900] lsr 0x60 / iir 0x0c > >> (previous two messages are repeated and printk_ratelimited()) > > > > Thanks for this information; I see I was wrong about the cause of message. > > > > I think what happens during startup is that on this silicon clearing > > the rx fifo (by serial8250_clear_fifos()) clears data ready but not > > the rx timeout condition which causes a spurious rx interrupt when > > interrupts are enabled. > > > > So caught between two broken UARTs: one that underflows its rx fifo because > > of unsolicited rx reads and the other that generates spurious interrupt > > without unsolicited rx reads. > > > > > >> When the issue is not triggered: > >> > >> [ 10.784871] lsr 0x60 / iir 0x01 > >> [ 10.788066] lsr 0x60 / iir 0x01 > >> [ 10.794734] VFS: Mounted root (nfs filesystem) readonly on device 0:13. > >> [ 10.801654] devtmpfs: mounted > >> [ 10.805169] Freeing unused kernel memory: 184K (807be000 - 807ec000) > >> (userland takes over after that) > >> > >> I have also displayed the IIR and LSR registers when the "too much fork for > >> IRQ" condition is triggered. > >> > >> In the serial8250_do_startup(), before the interrupt are unmasked at the end, > >> the IIR looks sane and UART_IIR_NO_INT bit is set. When stuck > >> serial8250_interrupt(), UART_IIR_NO_INT is cleared and the interrupt ID is set > >> to 0xc which is not handled by the kernel at this time (the Kirkwood datasheet > >> indicates that it is some kind of timeout condition from what I can gather). > > > > Yes, IIR == UART_IIR_RX_TIMEOUT is to used indicate that data is in the rx fifo > > but has not reached the rx trigger level yet. > > > > ATM, I'm not exactly sure if there is a safe way to clear the spurious interrupt > > from the interrupt handler. > > > > I'm fairly certain the only way to clear the rx timeout interrupt is to read > > the rx fifo, but I think this would race with actual data arrival. IOW, there > > might not be a way to determine if the data read is spurious or not. > > Yep, I see no safe way to clear the spurious interrupt [1] and no idea how to > keep it from happening (other than via the unsolicited RX reads in > serial8250_do_startup). > > Unfortunately, I think this means we'll have to revert Sebastian's commit: > > commit 0aa525d11859c1a4d5b78fdc704148e2ae03ae13 > Author: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx> > Date: Wed Sep 10 21:29:58 2014 +0200 > > tty: serial: 8250_core: read only RX if there is something in the FIFO > > which just means OMAP3630 will be limited to using the omap_serial driver. Reverting makes sense to me if it has caused a regression. Maybe Sebastian can update his patch to do this based on some quirk flag instead? Regards, Tony > [1] To clear the RX timeout interrupt requires reading the rx fifo even though > LSR[data ready] indicates no data. However, this could result in dropped data > if the data became available just before clearing the RX timeout. For example, > > CPU | Device > | > irq handler (simplified) | > | > read IIR | > is interrupt? yes | > read LSR | > is data ready? no | > is IIR == Rx timeout? yes | new data arrives > | rx_fifo[0] = new data > | lsr[data ready] = 1 > read RX and discard | > | > -- To unsubscribe from this list: send the line "unsubscribe linux-serial" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
