linux-kernel - Re: [PATCH] EG20T: Update PCH

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Date:	Fri, 12 Nov 2010 11:50:39 +0900
From:	"Tomoya MORINAGA" <tomoya-linux@....okisemi.com>
To:	"Alan Cox" <alan@...ux.intel.com>
Cc:	<andrew.chih.howe.khor@...el.com>, <kok.howg.ewe@...el.com>,
	<qi.wang@...el.com>, <yong.y.wang@...el.com>,
	"LKML" <linux-kernel@...r.kernel.org>,
	"Tobias Klauser" <tklauser@...tanz.ch>,
	"Feng Tang" <feng.tang@...el.com>,
	"Mike Frysinger" <vapier@...too.org>,
	"Kukjin Kim" <kgene.kim@...sung.com>,
	"Ben Dooks" <ben-linux@...ff.org>,
	"Greg Kroah-Hartman" <gregkh@...e.de>
Subject: Re: [PATCH] EG20T: Update PCH_UART driver to 2.6.36

On Thursday, November 11, 2010 8:42 PM, Alan Cox wrote:

> > > 
> > > The PCH DMA will have been included in the kernel anyway before this
> > > merge so you can remove the ifdefs and include it regardless
> > 
> > No.
> > PCH DMA have been already included at 2.6.36.
> > You can see "pch_dma.c" in drivers/dma.
> 
> So that means pch_dma is always in the kernel source tree as of 2.6.36
> - so the ifdef can go away ?

In case low traffic,
If UART driver is DMA mode only,
1byte data is also transferred using DMA.
This overhead is too big.
As a result, 1byte DMA transfer is slower than Non-DMA mode.
Thus, I think Non-DMA mode is also necessary.

> 
> > > tty = tty_port_tty_get(...)
> > > 
> > > and when finished tty_kref_put(tty);
> > > 
> > > Also tty may be NULL, if the port closed as you were doing this, so
> > > check the return from tty_port_tty_get and act accordingly.
> > 
> > I have a question.
> > Though I can't find any accepted UART driver uses these
> > functions(tty_port.../tty_kref...), What's for ?
> 
> We are gradually going through converting them all and I'm trying to
> make sure no new ones creep in that don't use tty_port_tty_get.
> 
> Basically it does this
> 
> tty_port_tty_get()
> 
> will return either a tty pointer with a reference (ie the tty will not
> be deleted while you hold it), or will return NULL if the physical port
> is no longer connected to a tty through close/hangup. It does all the
> locking internally to ensure this is done safely.
> 
> tty_kref_put()
> 
> drops the reference it obtained and after that point if the tty
> reference count hits zero it may be deleted.
> 
> It ensures this cannot occur
> 
> CPU1 CPU2
> 
> interrupt:
> tty = port->port.tty
> close
> port->port.tty = NULL
> kfree tty
> write to tty

Thank you for yor explanation.
I will add above.

> 
> 
> 
> > Many UART drivers accepted by upstream use the above macro, right ?
> 
> As far as possible the selection should be done at runtime, especially
> for PC class devices. Linux distributors want to build a single kernel
> for everything so having it runtime configured helps a lot.
> 
> 
> > > If you don't support CPARMRK then you should clear that bit in
> > > termios->c_flag so the caller knows it couldn't be set.
> > 
> > Sorry, I don't know CPARMRK.
> > What's CPARMRK ?
> 
> Sorry my fault. I mean CMSPAR
> 
> CMSPAR if set selects mark/space parity. If your hardware doesn't
> support this then do
> 
> termios->c_cflag &= ~CMSPAR;

eg20t doesn't support mark/space parity.
I will add above.

> 
> 
> > I will use dma_alloc_coherent.

I have modified and test.
Both  __get_free_page and dma_alloc_coherent work well.
I have a question.
Could you tell me the defferencies 
 __get_free_page(GFP_DMA) and dma_alloc_coherent ?

> > 
> > > 
> > > I assume the correct device in this case would be the DMA
> > > controller ?
> > 
> > Sorry, I can't understand your saying "correct device".
> > What does "correct device" mean ?
> 
> dma_alloc_coherent takes a device pointer which should be the device
> which is doing the DMA.

Yes, UART cooperates with DMA controller for UART DMA transfer.
Can you satisfy the above my answer ?
(I may not understand your intension correctly)

> 
> > Do you mean UART Rx/Tx interrupt Enable/Disable ?
> > If yes, the control hava been already implemented.
> > For example, in handle_tx (called from IRQ handler),
> > you can see pch_uart_hal_disable_interrupt().
> 
> I will take another look next time the patch is submitted. However if
> you are relying on disabling an IRQ source to avoid the interrupt
> running in parallel with another routine remember on a multithreaded
> CPU and on some hardware that it is possible for this to occur
> 
> 
> CPU1 CPU2
> Interrupt arrives
> Disable interrupt
> Interrupt routine runs Other code runs
> 
> 

Yes, I have used spin_lock_irqsave.
Do you mean we should implement another method for lock model ?


I will post the latst patch soon.

--
Thanks,

Tomoya MORINAGA
OKI SEMICONDUCTOR CO., LTD.
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