From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <5100FD2F.6080809@grandegger.com> Date: Thu, 24 Jan 2013 10:21:51 +0100 From: Wolfgang Grandegger MIME-Version: 1.0 References: <5100AB10.3030509@ebus.com> <5100F41C.2070006@grandegger.com> In-Reply-To: <5100F41C.2070006@grandegger.com> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai] rt_dev_write returns immediately on xeno_16550A List-Id: Discussions about the Xenomai project List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Doug Brunner Cc: xenomai@xenomai.org On 01/24/2013 09:43 AM, Wolfgang Grandegger wrote: > On 01/24/2013 04:31 AM, Doug Brunner wrote: >> I have a 16550A UART (actually 1/4 of a TI TL16C554) on the ISA bus on >> my WinSystems PPM-LX800-G SBC, and I'm finding that rt_dev_write returns >> control to the userspace process much earlier than I would expect, given >> how the kernel module is set up. (I'm using Xenomai 2.6.1 btw.) >> Fortunately it has a dedicated IRQ line, so I was able to put a logic >> analyzer on the IRQ, the TX line, and the RTS line (which my software >> manually controls to operate an RS485 transceiver). > > The 16550A driver does not support half duplex RS485 mode but I see > below that you it it's handled in your application. > >> I initially found that, with the TX FIFO size allowed to default to 16 >> bytes, rt_dev_write with a block of size 13 returns immediately. (I >> could tell this because my userspace routine sets RTS just before >> calling rt_dev_write and clears RTS immediately after, so it was visible >> on the logic analyzer.) This makes sense, because I could see that the >> 16C554 fires an IRQ immediately on the start of transmission (that >> violates the description of the TX IRQ in its datasheet, but that's a >> matter for TI). This would lead into rt_16550_interrupt, which would see >> that ctx->out_npend == 0 because the data was written to the FIFO in one >> go, and consequently fire out_event. The TX timeout was configured for > > rt_dev_write does store the data in a ring buffer and enable the TX > interrupt. When it occurs, the data is written to the FIFO and a > subsequent interrupt will signal completion when the FIFO is empty and > no more data in the ring buffer. > >> 100 ms, so rt_16550_write would be waiting on out_event, and >> rt_dev_write would return at that time. > > Yes, if the TX has not complete at that time. > >> However...I tried setting the TX FIFO to 1 byte via xeno_16550A module >> parameter, and found that rt_dev_write still returned almost >> immediately. This is what I don't understand, since I could see data >> being transmitted, and the IRQ line going high once for every byte. This >> means that rt_16550_interrupt MUST be getting called, and in turn >> calling rt_16550_tx_interrupt to move data out to the UART. For >> rt_16550_tx_interrupt to be doing so, out_npend must != 0, so out_event >> should not be getting fired until the very last character. > > What value does rt_dev_write return? > >> I did notice something odd: at the beginning of rt_16550_write, we >> initialize timeout_seq with ctx->config.rx_timeout. Later that timeout >> sequence is reused while waiting on out_event, which is naturally >> subject to ctx->config.tx_timeout. This seems like a possible source of >> error if tx and rx timeout are not equal--I didn't study the internal >> implementation of the timeout sequence so I'm not sure. However, I tried >> changing the references from rx_timeout to tx_timeout and it produced no >> change in behavior. > > The ctx->config.rx_timeout is used to lock (via mutex) the TX handling > against RX. > >> Do you know why rt_dev_write would be returning prematurely? I've >> attached the relevant userspace code snippets for reference. Note that >> RawInterface is a subclass of Interface, and higher level code will call >> RawInterface::Write. > > Not yet. An iPipe trace would help to understand the problem. Ah, I remember. FIFO empty does not mean that the transfer has been complete (on the bus). But that's what you need for toggling RTS for RS485. Wolfgang.