Design issues Keyspan serial PCI driver

Hi to you all,

after some struggling I now have a very first working version of the Keyspan SX
serial card driver, but I do not think the code I wrote is ready for publishing. To make a neat driver I have some design issues I want to share with you. Hopefully this will help me making the right decisions for the real driver.

The Keyspan SX cards use a PLX9050 as PCI bridge to connect to an ST16C654 UART. I found some patches for char/serial.c that support the UART directly. So this part of the problem was solved easily.

But Keyspan choose to map the UART to PCI memory space using the 9050, not PCI I/O space. Furthermore, for some unknown reason, the registers of the UART are separated 0x80 apart. Since char/serial.c uses I/O instructions to access the UART, I had to hack the driver to use readb() and friends. I am also multiplying the offset to compensate for the strange offsets.

I would really like to use the char/serial.c driver and keep it compatible with existing cards, because I do not want to write a complete driver myself for a UART that is supported by standard code. But then I have to decide runtime to use inb() and outb() or readb() and writeb(), since the driver also may be used for other cards that use I/O based access. Do you think that this would cause too much overhead in the interrupt handlers? It means that every I/O access will be slowed down by some extra memory references and a test.

Another point is that the serial structs use an int as the type for the port field. Since cannot change the struct in include/linux/serial.h to use u8 * as a type for compatibility reasons, I have to find some way to use the port as an offset added to some memory mapped base pointer of type u8 *, instead of using port as an absolute (I/O or mem) address. Just using the int port as a memory address value is dangerous, since it
won't work on 64 bits architectures. Do you think this is the way to
go: keeping compatibility with applications that expect port to be an int, while still be able to do memory mapped PCI access?

The good news of course is, if I can resolve the above issues, I have a driver that can access both I/O based and memory based PCI cards. The start of proper PCI serial card support in the standard serial driver?

You can find some more background info on my home page, <http://people.a2000.nl/omoerbe/>

Sorry for the long winding story, but I really would like some feedback,

Otto

Otto Moerbeek
[email protected]

[[ This message was sent via the linuxppc-dev mailing list.  Replies are ]]
[[ not  forced  back  to the list, so be sure to Cc linuxppc-dev if your ]]
[[ reply is of general interest. Please check http://lists.linuxppc.org/ ]]
[[ and http://www.linuxppc.org/ for useful information before posting.   ]]

Please also post your message on [email protected], since this is of interest not only for Linux/PPC development.

That would be great!

Greetings,

Geert

--
Geert Uytterhoeven [email protected]
Wavelets, Linux/{m68k~Amiga,PPC~CHRP} http://www.cs.kuleuven.ac.be/~geert/
Department of Computer Science -- Katholieke Universiteit Leuven -- Belgium

[[ This message was sent via the linuxppc-dev mailing list. Replies are ]] [[ not forced back to the list, so be sure to Cc linuxppc-dev if your ]] [[ reply is of general interest. Please check http://lists.linuxppc.org/ ]]
[[ and http://www.linuxppc.org/ for useful information before posting. ]]