On the transfer of data from the uart.c driver to the tty layer...

On the transfer of data from the uart.c driver to the tty layer...

[Alejandro C]

Hi all,

I'm working on a the uart.c driver to add support for SCCs in HDLC mode. The 
application I'm going to run on top will be sending Ethernet packets 
encapsulated in the data field of an HDLC frame. The idea was to make the 
buffer big enough so that a whole Eth. packet (max 1518 octects) could fit 
in just one buffer. I let the CPM interrupt on a frame basis and the problem 
I've come across is that the receive buffer in the struct tty_struct, where 
the tty layer collects the data from the driver 
(tty_struct.tty_flip_buffer.char_buf), is only 1024 bytes 
(2*TTY_FLIPBUF_SIZE). The easiest at this stage would be to make those 
buffers larger, but I don't know what sort of impact this might have... Any 
tips?
Any ideas on how to solve it in another fashion?

Help will be highly appreciated.

Regards,

Alejandro

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Re: On the transfer of data from the uart.c driver to the tty layer...

[Benjamin Herrenschmidt]

On Tue, 2006-09-05 at 14:56 +0000, Alejandro C wrote:
> Hi all,
> 
> I'm working on a the uart.c driver to add support for SCCs in HDLC mode. The 
> application I'm going to run on top will be sending Ethernet packets 
> encapsulated in the data field of an HDLC frame. The idea was to make the 
> buffer big enough so that a whole Eth. packet (max 1518 octects) could fit 
> in just one buffer. I let the CPM interrupt on a frame basis and the problem 
> I've come across is that the receive buffer in the struct tty_struct, where 
> the tty layer collects the data from the driver 
> (tty_struct.tty_flip_buffer.char_buf), is only 1024 bytes 
> (2*TTY_FLIPBUF_SIZE). The easiest at this stage would be to make those 
> buffers larger, but I don't know what sort of impact this might have... Any 
> tips?
> Any ideas on how to solve it in another fashion?
> 
> Help will be highly appreciated.

You should ask on the linux-kernel mailing list, possibly CC'ing Alan
Cox who seem to have put his hands a lot in the TTY code lately.

Ben.

Re: On the transfer of data from the uart.c driver to the tty layer...

[Dan Malek]

On Sep 5, 2006, at 10:56 AM, Alejandro C wrote:

> I'm working on a the uart.c driver to add support for SCCs in HDLC  
> mode.

Write a driver that does HDLC, please don't "update"
(complicate) the existing uart.c driver by adding
these features.  The uart.c is a UART driver, not
an SCC driver to be modified to your wishes.
Add another, configurable driver to do this.

> I've come across is that the receive buffer in the struct  
> tty_struct, where
> the tty layer collects the data from the driver
> (tty_struct.tty_flip_buffer.char_buf), is only 1024 bytes

Write a driver specific to your requirements that
has the proper sized buffers and management.

> (2*TTY_FLIPBUF_SIZE). The easiest at this stage would be to make those
> buffers larger, but I don't know what sort of impact this might  
> have... Any
> tips?

Yes, don't do this.  Those buffers are allocated from
a fairly small pool specific to the needs of the uart
driver.  Making these larger may quickly cause the
CPM drivers to run out of space.

> Any ideas on how to solve it in another fashion?

Write a driver specific to your requirements with the
proper buffer management for these sizes.

Thanks.

	-- Dan