2009-04-27 06:03:20 Read call from userspace fails in the chararcter driver

ThangarJ P (INDIA)

Message: 73274

Hi All,

I am working with uClinux 2008R1.5-RC3 distribution and BF533 Ezkit board.

i have wriiten a character driver for some purpose.

but i am facing an issue with read() call from usrespace at some times.

the read( ) call from userspace returns -1 sometimes. At the same time write and ioctl calls are working fine.

The read call from the usersapce wont call the kernal API which is mapped into the read in the fops table.It just returning -1 without call the kernal API.

The parameters in the read API are perfect and opening the driver also sucess with some id. the same id is passed to write and ioctl calls.they are working fine.

once the read API returns -1 then further read also gives always -1.It will work only after reboot the kernal.

With Regards

Thangaraj

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2009-04-27 07:28:43 Re: Read call from userspace fails in the chararcter driver

Robin Getz (UNITED STATES)

Message: 73281

Thangarj:

It could be any number of things. Did you check out:

https://docs.blackfin.uclinux.org/doku.php?id=driver_development

https://docs.blackfin.uclinux.org/doku.php?id=basic_read_write_functions

to test things out?

-Robin

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2009-04-28 02:24:38 Re: Read call from userspace fails in the chararcter driver

ThangarJ P (INDIA)

Message: 73337

Hi Robin,

ya..i have checked .

it is not happening consistently.some times its working proberly.

sometimes only i am facing this issue.

At that time it wont call the kernal API which is mapped to read api in the fops table.

if any other error means it should return from the API.

But here it never calls the kernal API.but it returning -1.

please provide your suggestions.

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2009-04-28 02:36:11 Re: Read call from userspace fails in the chararcter driver

Mike Frysinger (UNITED STATES)

Message: 73340

are you sure your userspace code is correct and is actually operating on an open file descriptor ? you really should review the standard methods for checking errors (like errno) instead of hunting down possibly bogus bugs.

you can also run things through strace

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2009-04-28 09:08:16 Re: Read call from userspace fails in the chararcter driver

ThangarJ P (INDIA)

Message: 73364

Dear Mike,

ya..I am sure the userspace code is correct and is operarting on open file descriptor.

because the same code is working most of the time.

some times only i am facing this issue.

ok. i will check through strace.

Thanks for your suggestions.

With Regards,

Thangaraj

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2009-04-29 01:46:08 Re: Read call from userspace fails in the chararcter driver

ThangarJ P (INDIA)

Message: 73401

Dear Mike,

Please find out my observations while run through strace.

SUCCESS CASE:

ret = read(s32_dvd_fd,pc_MsgBuf,ps16_NBytes) --> This is the call from userspace

s32_dvd_fd = 5; pc_MsgBuf = 0x74db4d; ps16_NBytes = 0x74db7c;

the sysem call for this read from strace command --> read(5, "\376r\6\2\0x\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0\0"..., 7658364) = 100.

FAILURE CASE:

ret = read(s32_dvd_fd,pc_MsgBuf,ps16_NBytes) --> This is the call from userspace

s32_dvd_fd = 5; pc_MsgBuf = 0x1eabb4d; ps16_NBytes=0x1eabb7c;

the sysem call for this read from strace command --> read(5, 0x1eabb4d, 32160636) = -1 EFAULT (Bad address).

In the Failure case also we are passing the valid address and correct device id.

please clarify why this is fail in this case?

With Regards,

Thangaraj.P

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2009-04-29 02:31:33 Re: Read call from userspace fails in the chararcter driver

Mike Frysinger (UNITED STATES)

Message: 73402

the 3rd argument to read() is a length, not an id. you cannot overload the read() function to do random things.

0x1eabb4d + 32160636 is 0x3d576c9 (or almost 64megs) ... you didnt post your kernel memory map, but i'm guessing that 0x3d576c9 is not a valid address thus the kernel is operating correctly.

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2009-04-29 02:50:24 Re: Read call from userspace fails in the chararcter driver

ThangarJ P (INDIA)

Message: 73403

Dear Mike,

The 3rd argument in the read is also a one pointer not a length.

The 32160636 is equal to 0x1eabb7c which is equal to the third argument.Thats a one pointer.

can we pass the third argument as a pointer in the read API?

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2009-04-29 02:59:09 Re: Read call from userspace fails in the chararcter driver

Mike Frysinger (UNITED STATES)

Message: 73404