From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <5100AB10.3030509@ebus.com> Date: Wed, 23 Jan 2013 19:31:28 -0800 From: Doug Brunner MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="iso-8859-1"; Format="flowed" Subject: [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: xenomai@xenomai.org 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). 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 =3D=3D 0 because the data was written to the FIFO in on= e = go, and consequently fire out_event. The TX timeout was configured for = 100 ms, so rt_16550_write would be waiting on out_event, and = rt_dev_write would return 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 !=3D 0, so out_event = should not be getting fired until the very last character. 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. 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. -- = Doug Brunner -------------- next part -------------- A non-text attachment was scrubbed... Name: rt-16550a-snippet.cpp Type: text/x-c++src Size: 2784 bytes Desc: not available URL: