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serial/pty: Remove the TODO comment for O_NONBLOCK

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since it is fully suppported now

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>
This commit is contained in:
Xiang Xiao 2022-02-22 13:45:51 +08:00 committed by Alan Carvalho de Assis
parent 0aa0022b12
commit 630b55feec
1 changed files with 0 additions and 44 deletions
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44
drivers/serial/pty.c
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@ -18,50 +18,6 @@
*
****************************************************************************/
/* TODO: O_NONBLOCK is not yet supported. Currently, the source and sink
* pipes are opened in blocking mode on both the slave and master so only
* blocking behavior is supported. This driver must be able to support
* multiple slave as well as master clients that may have the PTY device
* opened in blocking and non-blocking modes simultaneously.
*
* There are two different possible implementations under consideration:
*
* 1. Keep the pipes in blocking mode, but use a test based on FIONREAD (for
* the source pipe) or FIONSPACE (for the sink pipe) to determine if the
* read or write would block. There is existing logic like this in
* pty_read() to handle the case of a single byte reads which must never
* block in any case: Essentially, this logic uses FIONREAD to determine
* if there is anything to read before calling file_read(). Similar
* logic could be replicated for all read cases.
*
* Analogous logic could be added for all writes using FIONSPACE to
* assure that there is sufficient free space in the sink pipe to write
* without blocking. The write length could be adjusted, in necceary,
* to assure that there is no blocking.
*
* Locking, perhaps via sched_lock(), would be required to assure the
* test via FIONREAD or FIONWRITE is atomic with respect to the
* file_read() or file_write() operation.
*
* 2. An alternative that appeals to me is to modify the contained source
* or sink pipe file structures before each file_read() or file_write()
* operation to assure that the O_NONBLOCK is set correctly when the
* pipe read or write operation is performed. This might be done with
* file_fcntl() or directly into the source/sink file structure oflags
* mode settings.
*
* This would require (1) the ability to lock each pipe individually,
* setting the blocking mode for the source or sink pipe to match the
* mode in the open flags of the PTY device file structure, and (2)
* logic to restore the default pipe mode after the file_read/write()
* operation and before the pipe is unlocked.
*
* There are existing locks to support (1) destruction of the driver
* (pp_exclsem) and (2) slave PTY locking (pp_slavesem), as well as (3)
* locks within the pipe implementation. Care must be taken with any new
* source/sink pipe locking to assure that deadlocks are not possible.
*/
/****************************************************************************
* Included Files
****************************************************************************/