linux-kernel - Re: [PATCH] hvc_console: returning 0 from put

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Message-ID: <4AD77007.2070706@freescale.com>
Date:	Thu, 15 Oct 2009 13:55:03 -0500
From:	Timur Tabi <timur@...escale.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>
CC:	Scott Wood <scottwood@...escale.com>, linuxppc-dev@...abs.org,
	brueckner@...ux.vnet.ibm.com, Alan Cox <alan@...rguk.ukuu.org.uk>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] hvc_console: returning 0 from put_chars is not an error

Christian Borntraeger wrote:
> Hmmm, if we are ok with having both options, we should let the hvc backend 
> decide if it wants to drain or to discard.
> 
> If we just busy loop, it actually does not matter how we let hvc_console react 
> on 0, as long as we adopt all backends to use that interface consistent.
> 
> On the other hand, backends might want to do special magic on congestion so I 
> personally tend to let the backend loop instead of hvc_console. But I am really  
> not sure.

The reason that we're asking for this change is that I have an hvc client driver that drops characters during heavy printk() output.  hvc calls my driver, but the output buffer is still full, so the driver just returns 0.

If I add a spin-loop in the driver, then we don't need to change hvc, but now the driver is blocking during user-space print operations (via hvc_write and hvc_push).  

-- 
Timur Tabi
Linux kernel developer at Freescale
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