Hi Claudiu,
-----Original Message----- From: Claudiu Beznea claudiu.beznea@tuxon.dev Sent: 18 December 2025 07:44 Subject: Re: [PATCH v5 2/6] dmaengine: sh: rz-dmac: Move all CHCTRL updates under spinlock
Hi, Biju,
On 12/17/25 18:16, Biju Das wrote:
Hi Claudiu,
-----Original Message----- From: Claudiu claudiu.beznea@tuxon.dev Sent: 17 December 2025 13:52 To: vkoul@kernel.org; Fabrizio Castro fabrizio.castro.jz@renesas.com; Biju Das biju.das.jz@bp.renesas.com; geert+renesas@glider.be; Prabhakar Mahadev Lad <prabhakar.mahadev- lad.rj@bp.renesas.com> Cc: Claudiu.Beznea claudiu.beznea@tuxon.dev; dmaengine@vger.kernel.org; linux- kernel@vger.kernel.org; Claudiu Beznea claudiu.beznea.uj@bp.renesas.com; stable@vger.kernel.org Subject: [PATCH v5 2/6] dmaengine: sh: rz-dmac: Move all CHCTRL updates under spinlock
From: Claudiu Beznea claudiu.beznea.uj@bp.renesas.com
Both rz_dmac_disable_hw() and rz_dmac_irq_handle_channel() update the CHCTRL registers. To avoid concurrency issues when updating these registers, take the virtual channel lock. All other CHCTRL updates were already protected by the
same lock.
Previously, rz_dmac_disable_hw() disabled and re-enabled local IRQs, before accessing CHCTRL registers but this does not ensure race-free access.
Maybe I am missing some thing here about race-access:
local_irq_save(flags); rz_dmac_ch_writel(channel, CHCTRL_DEFAULT, CHCTRL, 1);
After local_irq_save there won't be any IRQ. So how there can be a race in IRQ handler.
My point was to address races that may happen b/w different cores trying to set CHCTRL. E.g.:
core0: take the IRQ and set CHCTRL core1: call rz_dmac_issue_pending() -> rz_dmac_xfer_desc() -> rz_dmac_enable_hw() -> set CHCTRL
For that you could use a new lock. Currently all the calls are serialized using vc_lock In irqhandler and irqhandler unnecessarily keeps spinning as the chctrl register is the only shared Data in interrupt handler.
However, looking again though the HW manual, the CHCTRL returns zero when it is read, for each individual bit. Thus, there is no need for any kind of locking around this register. Also, read- modify-write approach when updating settings though it is not needed.
But still you need a lock other than vc_lock as CHCTRL is shared register between IRQhandler and tasklet/process context
Cheers, Biju