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Date: Thu, 15 Oct 2009 13:55:03 -0500
From: Timur Tabi <timur@freescale.com>
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To: Christian Borntraeger <borntraeger@de.ibm.com>
Subject: Re: [PATCH] hvc_console: returning 0 from put_chars is not an error
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Cc: Scott Wood <scottwood@freescale.com>, linuxppc-dev@ozlabs.org,
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	Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
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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